Go for one 80Gb as a minimum. You're not going to be able to install anything on the 80Gb one other than the OS (Windows), Photoshop (CS4), Adobe (Full Version 9), Microsoft Office 2007 Complete, you mail configured, into Outlook, a good antivirus, a few good browsers, a few music players, ..... that's about it since you need atleast 20Gb free for data which you might keep shifting onto a 7200RPM drive .....
That's about the basic set of programs which I've listed, I guess any standard user has these on to fool around with.
So that's about it, if you over crowd the drive it's going to slow down the pc, and that you don't want.
I give one up to Cobot for the SSD plus HDD idea