Just for the OS & Apps, with files being stored elsewhere? SSD it is then.
Back in the day when they were just released (man that makes me feel old!) I had a 74GB WD Raptor as the OS & Apps drive, and 300GB Seagate for files. 10,000RPM was a serious improvement.
These days it's a 80GB Intel G2 SSD for OS & Apps, and RAID1 WD Cavair Blacks for files.
Interesting that in neither of those setups have I felt cramped by only having a small C:\ drive.