If you're happy with the MBP performance, I'm sure you'll be very happy with an iMac. It's the fastest and smoothest-operating computer I've ever had, much snappier than my dual-core Pentium 3.0GHz Vista PC in day-to-day use.
Adding RAM is cheap and easy. Just buy good name-brand Kingston or Crucial RAM for under $50/GB. Adding a single 1GB stick is enough for most tasks. Don't get the low-end 20" if you care about 3D performance at all, it's got an inferior Radeon 2400 graphics chip. The mid-range 20" and 24" have a nice mid-range 2600 PRO (actually a lower-clocked 2600XT). And the 24" screen is clearly superior. I have the 20" and the brightness is uneven from top to bottom, but I don't notice it most of the time (web surfing, using iTunes, playing games). The 20" screen is bright, clear, and sharp though, especially after calibration which is almost a necessity. If you do a fair bit of photo editing or graphics work, I'd get the 24".
And don't listen to Mac6272, he had all sorts of condensation problems with his iMacs. He's either extremely unlucky or lives in the middle of a swamp. Perhaps both.