I have 6 HDDs in this system. 2x750 GB, 1x320 GB, 3x250 GB. I voted for 750 GB because I hate my 250 GB drives and the 320 GB one. I should replace them all with one 1 TB drive or two 750 GB drives.
I just built a new system, and as I needed more disk space and only had a single interface for two IDE devices, I went for a 1 TB Seagate SATA2. Took a while to figure out AHCI, but it's now working fine.