shain... reading will answer your questions... but ill save you some work as you are new...
There arent any CAIs for the coupe... K&N is coming out with one but i dont think its hit the streets yet and its gonna tip the scales at around $300...
I personallly have the 2.5 with the fujita... why? because its the cheapest one around and the difference between them is something like the difference between 1080p and 1080i or the difference btwn hd dvds and blue ray... something thats essentially the same...
All HP gains claimed by the intakes are somewhat "hot air" and what you should expect from a WAI (warm air intake, no one wants a HAI lol) is a mean badass sound... you may notice an increasein performance, you may not.
I got my fujita on ebay for $150 delivered.
Also, it was only a year ago i was "green" like you and had no idea what i was doing or where to look, and now im a moderator on the most awesome nissan site on the net. You will learn quickly... Why? Because we are here to help! And the answers are all here! (or we are working on them)...
So from HIDs, to interior LEDs to intake and exhaust... even turbos... look around here and you will be satisfied... and if its not covered, bring it up!
http://forums.altimas.org/zeromainRead the stickys, go thru the first 2 pages of threads in each category.. you will be a "modder" in no time!