I was wondering what is the best thing that I could buy for around 300-500 dollars, I'm looking for something I could get for christmas. If you guys could tell me what I could by for that price which will give me the best performance increase.

 

Convertible, NA, Turbo II ??

You could buy a few guages, pillar mount gauge pod.
You could buy a used leather interior
You could buy a hatch cover if you car originally came with one
A new exhaust
etc.. etc...

 

If the car is stock right now, and regardless of which model it is...

Exhaust system!

 

it all depends on the level of mod's it has already...

 

intake then exhaust choices i.e. header and presilencer or downpipe and presilencer or catback these are your options but you may be limited money wise on the exhaust options. I have a custom setup from my manifold back and it cost me a total of 210 dollars 2.5 downpipe to 2.5 catback with an apexi dunk muffler (single). You will find stuff is not cheap for an RX-7, that is because there are not alot of companies that make stuff for our cars...but the companies that do make very good high quality stuff.