Since you are on a budget (like me), the main question is what version of adobe CS will you need to have for the next several years. The cost of that can cost more than the Mac. Assuming you are going to get a newer version of adobe CS, look at some prices on those and get a Mac that will run it well.
For example if you go for CS 4, 5, or 6, a new Mac Mini would be best (assuming CS 4 runs fine on ML, correct me if it doesn't). If you can only afford CS 2 or 3 then you would be running a used G5 or Intel Mac with Snow Leopard. You can get dual G5s cheap with a couple of big HDDs but I wouldn't count on it lasting you too long. So go Intel if you can afford it. Minis are a good deal.
If I can get a used mini 09' or so... Will CS2 & 3 run well on Leopard? Snow Leopard? I am not in a position to upgrade the software right now and need to use the CS2/3 I have now ALL THE TIME... so, if it might not work well... I may be stuck with PPC until I can afford the new software. ?