IMac :: Making Specs And Size Decisions?

I'm about to buy an iMac, and need some help in the final part of my decision-making.

Many threads on this site have been very helpful for answering most of my questions, but now I just need some tailor-made advice for this last bit. Thanks in advance!

Questions about specs first...

I'd use the comp for:the regular stuff (internet, email, word docs, music)
quite a bit of graphics work in programs such as Photoshop, Illustrator, and Painter
Bootcamp for a few graphics-intensive games (Cabal, Assassin's Creed, ...Maplestory? lol)
Bootcamp for graphics programs that aren't available for the mac os

So, on the following 2 specs, please tell me which option would be good, but not redundantly powerful.

Core2Duo/ i3/ i5/ i5 Quad/ i7 Quad

ATI Radeon... 4670 with 256MB/ 4850 with 512MB/ 5670 with 512MB/ 5750 with 1GB

I'm also trying to decide on size. 21.5" is actually a fine size for me, but the maximum specs for 21"s are 3.6GHz, i5, 5670 with 512MB. If this is enough ("comfortably enough"... I don't want "just enough") for my needs, then great. Otherwise I'll have to look at the 27"s for better specs-- unless you know how to beef up the 21" while keeping it cheaper than the 27"s.

Price isn't tooo much of an issue. I can afford up to a 27", 2.93GHz, i7 Quad, 5750 with 1GB. But if my maximum uses require significantly less than that, then I don't want to pay for power that's so far beyond necessary.

"Buy the best you can afford" is tempting, but in the end I'm a little too pragmatic and stingy for that.

Any advice/pointers/stories from experience would be welcome!