Heres what I can comment on for what I have so far
Gemini catback exhaust - biggest power gainer by a good number, plus it adds a nice sound. Its the quietest exhaust from what ive heard, and when I go to the meets, it is my favorite sound.
Swift springs - This was a great suspension mod, handling improved a good deal but lowered the car where I scrape almost everywhere. Railroad tracks I have to go 5 mph over, but the lower plastic things take a brunt of the force.
Ichiba rear camber arms - Just get these with the springs, the rear camber will get knocked outta whhack, and unless you want some SERIOUS camber changes, go with the ichiba. It costs about 60 bucks cheaper and does the same thing, but can only adjust 2 degrees both ways, instead of SPC's 3 degrees. Ichiba is perfect.
Stillen front and rear sway bars - By far the most noticeable suspenison mod. It took my shop an hour to do (I dont do any work myself, I figure i went to school and got a good job to make enough money to have somebody do it for me
) and I had them put it on the stiffest setting.....My car is like riding on rails now its soooooo great. If I could do it all over again, i would do stillen sways before the springs.
Mishimoto post maf hose/K&N filters - I was not expecting big gains from this. Mid range is a definitely a little peppier and the sound is a little better. When you rev it to about 5000 you hear the noise. But my exhaust overrides it a bit.
Now youll hear people using Berk/invidia/stillen test pipes/HFC's, but personally I wouldnt mess with the cats. Ive spoken to numerous people and they all claim it added very little power but mainly noise to the exhaust. And to me, IMO, isnt worth cost. Its REALLY noisy.
You already know my thoughts on stillen intakes.
Headers, again, I dont see many people using because its very low horsepower adder. Plus its a PITA to install and it quite pricey.
Supercharger - Now this would obviously be the best way for power. Kirkster is the only one I know of thhat has one of these. But hes also a master mechanic and if something breaks, he can fix it.
I bought all of my parts from Jomer. Hes on the forums here and will always get you the best prices. FBnissan is his name on the forums here so dont shop online before consulting forum vendors for prices.