I run Photoshop (full size) on an 8 year old Mac G4, with 768 RAM (mismatched 256 & 512 chips). I did upgrade the processor not long ago - from 400 to 1 gig, and had to upgrade my graphics to accomodate my new 23" Apple Cinema Display.
Would more RAM and faster processors be better? Maybe ...
Faster? Of course ...
Necessary? No.
A lot depends on your workflow, how you choose to work, and how patient (or impatient) you are.
Having the latest, greatest, fastest ... whatever! ... is nice. But it's hardly necessary.
Before you buy the software, check the spec requirements. If your current computer meets the minimums, don't worry about it.