Suggestion:
I realize this is the "iMac side" of the forums, but as a "switcher", have you considered the Mac Mini, as well?
You can keep your present display, keyboard, mouse if you wish.
You didn't specify a price range, but the 2.3ghz i7 Mini is a very nice little "box". Perhaps its only weakness is on the graphics side -- it probably wouldn't satisfy you if you're a gamer. But the HD4000 graphics on mine have never choked when running non-gaming stuff...