IBM Deskstar 75GXP, I have the 30gig variety and I love it.
Fastest IDE hard drive around right now, it's also very reliable and quite quiet, available in 15,20,30,45,60,75 gig sizes.
The Maxtor DiamondMaxPlus 45 is a good choice as well, but I prefer the IBM.
RPM is how fast the drive spins, faster spinning reduces aggregate seek time and increase transfer rate. Therefore the higher the better, so 7200rpm is better. 5400rpm is probably quite a bit cheaper though.