Would or could any maxima stock beat a stock 2000 Grand Prix GTP? Also, what about a Slightly modded Toyota avalon? How bout a prelude? Are there any cars I really need to look out for that are stock sleepers?

" I really need to look out for that are stock sleepers?"

No u just need to throw some mods on the GTP :)

What would be the best mod? 3.4 pulley and pcm? If so, which pcm to purchase?

For starters:

Modular pulley system w/ 3.4
DHP PCM
Intake
Ubend delete
3"Down pipe
180 tstat
Autolite 605's or 104's

How important is the Intake, Ubend, and Down pipe? If I get a 180stat, Autolite's, DHP, and 3.4 pulley what would typical times be for 0-60 and what HP would I be putting out? I just bought the car and really like it!

Those 3 help the engine breath better.

W/ 3.4- intake- exhaust- plugs I dynoed 220

Going up against the newer Maxima is really a drivers race in the quarter stock for stock. After 100 or so a stock gtp gets embarassed. You will need some rockers or a cam if you want to whoop ass on the highway.