WD or Seagate 7200.10
AnandTech: Western Digital and Seagate: 320GB Grudge Match . A very good review of the Seagate 7200.10 320GB. If you can believe it this drive is just $99 right now as is the competing Western Digital drive. The performance results you can read, but these are both screamer drives with the WD being slightly quieter.
The big difference is the five year warranty for Seagate. I've had about 30% of my drives fail, so that's a big issue. Right now I have three of the 7200.10 750GB. They are really expensive at $350 compared to a 320GB at $100 but I like them because of the density. I can get 1.5TB into a small desktop.
Only note is that if you are going to have drives running at any speed make sure to have a front case fan. This drops the temperatures of the drives by 20% and is a big factor in failure IMHO. I use the expensive Nexus Silent Fans. They are bit 5.25" fans and cost $15, but are so quiet. I have one front case fan in one of our machines and will put them in the rest shortly.