well, here is what I'm doing: I have 00 accord lx sedan and I also been thinking about adding some power. But anything major costs major $$: engine swap is like 4k, turbo 4-5k. I was thinking about selling my car and buying used s2000 for like 20k, but I'll have to put huge down payment and I am probably gonna get killed on my insurance. So, guess what I came up with after all this financial considerations: in two month I am buying a motorcycle.
Nice, used Kawasaki 600 will be just about k3.5. So, I might sound stupid, but if you are chasing speed and power( like I am) bike is the only smart way to go. Needless to say, $4000 bike will outperform any car under $200000 and some over that. I am still gonna do some mods on my car like i/h/e/p and suspension work. But thats about it. Also, one more thing: when you put money into the major mods(like turbo, swap) you will never get it back. $5000 worth of upgrades may add about 2k to value of the car.