Ive already designed an intake to bring in more cold air and I removed the factory mufflers on the rear and replaced them with a set of Flowmasters I had on my old chevelle. I havent went with the full exhaust cause my wife will be ticked If it gets any louder and to be honest I like the db level its at. It sounds a lot like the infinity g35 in terms of loudness. Any fuel mods I can make? Perhaps a computer reprogramming(Prob more than I want to spend)
The Infinity and Zcars are making nearly 300hp from a very similar engine. Is there and Air Flo sensors or other bolt on goodies that can be used off these cars that will work on the altima with out confusing the ECM.

MSD Ignition Box is not very expensive (gives you more power)
MSD Blaster Coil is a great partner (gives a much hotter spark)
Bosch Platinum 4 spark plugs (4 separate grounding electrodes give biggest spark possible)
Stay with stock spark plugs tough.
Maybe a Venom 400 Chip ($$expensive$$)

MSD Ignition Box is not very expensive (gives you more power)
MSD Blaster Coil is a great partner (gives a much hotter spark)
Bosch Platinum 4 spark plugs (4 separate grounding electrodes give biggest spark possible)
Stay with stock spark plugs tough.
Maybe a Venom 400 Chip ($$expensive$$)
Will I see or be able to tell a diff with these upgrades or will a full header system create greater gains. I think I could probably get a set of headers for around the same price as an ignition upgrade, but I will need to find a muffler system that keeps my db's low yet flows more air. Dang! Horsepower gains become hard to get when you are concerned about noise. Thanks for the post I will definitely look into the platinum plugs for now.

MSD Ignition Box is not very expensive (gives you more power)
MSD Blaster Coil is a great partner (gives a much hotter spark)
Bosch Platinum 4 spark plugs (4 separate grounding electrodes give biggest spark possible)
Stay with stock spark plugs tough.
Maybe a Venom 400 Chip ($$expensive$$)

get the fuck out of here. you know nothing of how cars work. MSD will do absolutely nothing on a modern stock car that a tune up wont do. bosch plugs are a no-no for nissans. get some good NGK iridiums. venom chips blow ass.

for some easy nonturbo power, get a well designed intake, 2.5" mandrel bent exhaust, and some good headers. anything you can do to let the engine breathe better. also, you could try a small nitrous shot (no more than 50hp) or a chipped or reflashed ECU(from a good NISSAN tuner, like jim wolf technology). if you want to get a little further into your engine, you could get some street grind camshafts and a 3 angle valve job. you could go turbo or supercharger, but its expensive and much more complicated.

oh and if your worried about sound, get an exhaust with a full muffler, such as these:
http://www.apexi-usa.com/product_exhaust_detail.asp?id=176&pageNum=1
http://www.stillen.com/product_detail.aspx?sku=508240
http://www.raceinspired.com/ps-94-31-greddy-street-performance-sp2-catback-dual-exhaust-nissan-altima-35l-2002-up.aspx

or probably the one that would please you the most is the borla exhaust. it has a couple silencers.
this link is for the whole catalog, 56k beware. its almost 8mb. the altima is on page 29.
http://www.borla.com/media/2005borla_app_guide2.pdf
if this link doesnt work, use the one below and click on download the PDF catalog, and go to the bottom of page 29 to see it
http://www.borla.com/catalog/

Thanks for the time and research Man! Very helpful links. That stillen system is a must have. I would love to put a turbo system on the car but I couldn't run that much boost since the car has high compression pistons, but If I still have it when its time for a rebuild who knows. But then its a FWD that kinda defeats the purpose since the thing isn't going to hook up when you start hitting the 300hp range. Oh well, maybe I should just look for used g35 to spend money on the big stuff. The Advice you have given me will give me an edge over most of the altimas in the area witch is my biggest concern. If its no bother, explain what the 3 angle valve job is all about. I here that term in ATV racing but i'm not sure whats changed to provide better airflo.

3 angle valve job smooths the angle in which air enters the cylinder, allowing the air to move faster and with less turbulence. it also helps the valves seal better, but thats just a side effect. a valve job combined with porting and polishing the head will provide some decent gains throughout the powerband, but mostly in the upper rpm ranges.
this is a good explanation:
http://www.hanzenginehouse.com/HANZENGINEHOUSE/pages/Valveseat_closeup.html

3 angle valve job smooths the angle in which air enters the cylinder, allowing the air to move faster and with less turbulence. it also helps the valves seal better, but thats just a side effect. a valve job combined with porting and polishing the head will provide some decent gains throughout the powerband, but mostly in the upper rpm ranges.
this is a good explanation:
http://www.hanzenginehouse.com/HANZENGINEHOUSE/pages/Valveseat_closeup.html
I can definitely see where it would help. If a person had a limited supply of aftermarket parts, like a larger camshaft, this would be a good alternative for more air-flow. It basically takes the sharp corner the air would normally have to flow around and allow the air a more direct path into the combustion chamber. Clever Idea! If a head gasket ever goes and I have an excuse to take it off. You can bet that babby will get a port job. Ive always heard that the heads is were your horsepower its in a NA engine and it only makes sense. I here its wise not to polish the intake as the rough edges on the wall helps to atomize the air and fuel. Do you agree? Never mind that statement the altima has direct fuel injection doesn't it. Got to go, talk more later.

NissanPerformanceMag.com (http://www.nissanperformancemag.com) had an Altima 3.5 SE project car (look under archived projects). They did everything from cold air intake and headers to cams and ECU; they also did suspension mods. Thier installation of an AEM cold air intake netted a gain of 5 hp from 2k to 5k rpm, and peak gain of more than 8 hp - dyno tested. This seems like the easiest thing to do (along with a cat-back exhaust).

On any engine the key is more air in and more exhaust out, and on modern engines the computer is smart enough to take advantage of increased airflow (up to a point) without going to computer mods. Of course at some point cams and computer chips will allow the engine even better airflow!

I would bet (but I don't know) that the only major difference between the Altima 3.5 and the Infinity is intake and exhaust flow. The same basic engine only makes 205 hp in the Quest, but you can imagine the intake/exhaust are pretty restrictive in a minivan...

for the most part your right, the 350z and g35 have different intake manifolds and somewhat freer flowing exhaust, but also have a differently tuned ECU that makes much of the power. im pretty sure the intake manifold for the Z wont fit in the engine bay correctly, otherwise it would be a nice mod for altima owners. plus theres a company that sells a spacer that goes under the upper manifold cover on the Z and G35 that gives some more hp. all this is speculation of course, but you could do some research and see what it would take to use this manifold.

and yes, some rough surface in the intake runners is better for standard injection engines, your 100% correct in your reasoning. but direct injection doesnt need to worry about this. im not sure what the inside of an altima intake manifold looks like, but usually on v6 engines theres some material that can be taken off with a hone, you could have a machine shop look into this if you ever get around to head work.