If I didn't game, I'd probably still be fine with the Athlon XP 1600+/Radeon 8500 machine that I was using when I was a senior in high school. (Mind you, I'm only a sophomore in college right now )
Since then I've bought a Radeon 9700 Pro and then a 6800GT; for CPU's I bought an Athlon XP 1700+ (that's been running at 2400MHz ever since I got it). I've had that CPU since August 2003, and I'm finally about to upgrade to A64 soon - mostly because my CPU is holding back my video card quite a bit, I'm sure. Oh, and I also now have a 1680x1050 20" Widescreen LCD, so a pretty high-end computer is necessary to drive the latest games at its native resolution. It's a vicious cycle, really.
I wouldn't have it any other way though. (Also, the cost is not as bad as it could be, since my friends/brother tend to buy my older stuff as I upgrade.)