It's a stock standard 3.5" 7200rpm HDD. iMac's have used them since at least 2003 except for the very first G5 iMac which used a 5400rpm 40GB drive at the very bottom end. The rest of the drives were 7200rpm.
I know it doesn't exactly relate to the model you're purchasing, but I thought I'd mention that my 3.06GHz/1TB iMac shipped with a Western Digital Caviar Black, which I was pleasantly surprised about