Western Digital Caviar Green 2TB (WD20EARS).
First step in hard drive space upgrade I'm planning in this PC. Second step will be another 1TB 7200rpm drive for a RAID0 stripe with the current system drive, and the (potential) third step will be an SSD to speed up the system even further; the system and most used software will then go onto the SSD, games and all other software onto the 1TB RAID0 stripe, and data goes onto the today's newcomer 2TB (which will probably also get a twin for a RAID1 mirror at some point).