Depending on what you want to do, there are several ways to go. If you are just daily driving it, I would find parts that would keep or give you some low end to mid range torque. Once you start losing it and gaining high end, you'll get a little pissed during daily driving as to the slugishness through 4k rpm.

cheap things I can think of are k&n filter... ummm haha I dunno. Nothings cheap really unless you buy ebay or do some weight reduction such as stripping out your whole trunk, removing a/c, so on and so on. Though you could spend money on things that will improve looks and performance at the same time ... i.e. Lightweight wheels.

How much are you looking to spend anyways?