Gregg: if all you plan to do is "dust spotting" and you don't plan to rack up dozens of layers, you'll probably do just fine with a 24" iMac. Definitely get at least 4GB of RAM—I think that may be the current minimum (yay!). In another few months, you'll probably be able to get 2x4GB modules for much less than you currently can, so I'd try to avoid paying for 8GB right now.
If you haven't already, I also recommend you familiarize yourself with Lloyd Chamber's
Macintosh Performance Guide. Some of his recommendations may be overkill for you, but at least you'll know exactly what the issues are.