If you're building a dedicated gaming rig, get a WD Raptor hard drive. Excellent performance, but you pay the penalty in capacity. I use a 72g raptor on my gaming pc, in 2 partitions. 1 partition for windows, 1 partition for installed games, and a seperate, much larger hd for mp3s, movies, etc.
My load times are half that of people with faster pc's but slower HD's. I didn't expect that much of an improvement, but I was impressed the day I started using it, and it's been 100% reliable for over 3 years now.