changes...

if u change the crank pulleys and all like on the 3sx site they have pulley sets that are lighter and look smaller then the stock ones will that effct how the engine runs wiill i have to change other stuff too??

There are 2... Unorthodox racing is a underdrive pulley which slows down all the assesories which I would not recommend unless you don't really turn anything on in your car...
There is a 3sx one which is OEM size but is 10 lbs lighter and does not have the harmonic balancer on it.
I wouldn't really worry about the harmonic balancer as the flywheel/clutch plate on the other side is the roughly the double to triple the mass but does not have one, and the belts will do the same job as the harmonic balancer (just be prepared for premature belt wear).
One thing you will encounter is that your car RPMs will be erratic as the ECU tries to remap the fuel curves because the RPMs drop a lot faster because of the lack of rotating mass in the engine.

One thing you will encounter is that your car RPMs will be erratic as the ECU tries to remap the fuel curves because the RPMs drop a lot faster because of the lack of rotating mass in the engine.

but, as the RPMs will drop a lot faster, so will they rise/rev a lot faster as well! so in principle, this would have the same effect as a lightened flywheel... just the decrease in rotational mass is one the opposite side of the engine from the flywheel. (and there's that whole debate about harmonic balancer...necessary or not? but there have been no ill effects yet from the use of a lightened (stock size) crank pulley.)

but, as the RPMs will drop a lot faster, so will they rise/rev a lot faster as well! so in principle, this would have the same effect as a lightened flywheel... just the decrease in rotational mass is one the opposite side of the engine from the flywheel. (and there's that whole debate about harmonic balancer...necessary or not? but there have been no ill effects yet from the use of a lightened (stock size) crank pulley.)

LOL!!! I forgot about adding why you would do it...

There are calculations that you can do on how much dynamic HP (as in acceration and not straight pull tests) You gain by decreasing the engine's moment of inertia. I am not going to run the numbers because I am not nearly bored enough to want to, but someone calculated it for a Nissan 240sx and decreasing the flywheel weight by 10 pounds equils about 40 HP in first, 12 HP in second and 4 HP in third (you will notice that it's geometric because it's based on how fast the gear changes in RPM... The faster the change the greater the gain).

Changing the Pulley won't give you the same gains as using a lightened flywheel because the diameter of the pully is smaller (hence less moment of inertia) but it does make a difference.

See? It does pay to pay attention in physics class!