I got the 4GB RAM upgrade from Apple, but put in a Hitachi Travelstar 7K320 320GB that I already had in my old Acer laptop. I was initially going to put my 500GB Scorpio Blue in instead, but after reading the about the problems with it on here, I decided against it. It's staying in its USB enclosure. My friend put a 500GB Hitachi Travelstar 5K500.B in his 13" UMB and loves it.
Oh, and just to be clear, the 5K500.B is the newer version with 2 250GB platters and the standard 9.5mm drive height, while the older 5K500 used 3 166GB platters and was 12.5mm tall. The newer version is preferable since fewer, denser platters means lower noise, reduced power requirements and higher transfer rates, not to mention the hassle of the 12.5mm drive just barely fitting in there.