To see an SSD here since one folder alone(Steam and Steam mods) comes in at 358gb! Some good news for SSD fans however looking for more drive space. Micron unveils its first 1TB SSD -- for under $600 - Computerworld
Gradually other companies will also start seeing prices drop on the larger models as well as time goes along. If you are one who puts a lot on a single C volume like VMs and other large apps a 64gb would be too cramped to consider.
For a basic clean install and a few programs however with a second larger drive for storage and backup you shouldn't have a problem. It depends mainly on how much you would normally see on a single drive if that was all you had say on a laptop or single drive desktop when planning out how large a drive you would have to have.