first off...welcome to han:wavey:

a few basic ideas that you can do just to kick it up a little bit AS WELL AS increase the gas mileage of the car is the basic intake, header, exhaust (I/H/E)

intake - AEM Cold Air or Ram Air (depending on the puddles in your area)*sucks in higher volume of air, keeps the engine cool, more efficient than the stock one air induction system*
Header - D/C 4-2-1
*allows more air to leave the engine faster, therefor letting it breathe better*
Exhaust - it's up to u, a few good brands are apexi, greddy, HKS, look for what flows, sounds, looks best for YOU
*allows the exhaust gasses that have just left the header, continue out the car at the same speed, also allowing the car to breathe better

all of these are for better "breathing" which in short will improve the already great gas mileage of the 3rd gen integras

also if your buying it for under $5k
- make sure that the 90k mait. was done
- it's possible to find them that cheap, but the condition will usually be not too great, so doing a full tune-up is suggested,
- obviously the full tune up comes before aftermarket parts

thats all i can think of right now...but im tired...maybe more will come to me later