@TheDeadWake Maybe the light-duty stuff, but I’ll take my cheap GT240 card against the GT320M mobile graphics chipset in the MB Pro any day, at least so long as 767 is greater than 364.
The iMacs clock in at 137, 774, and 1077; the low-end one is pretty crappy (worse than my netbook), the two middle models are on par with my box that cost one-third of what Apple wants, so the top-tier iMac (also the only one with a Core i5 instead of yesteryear’s Core2 Duo, though no match for an i7 or a Xeon) is the only one that can really compete. Even then, it can’t match a good dual-card setup like you need for the really heavy-duty stuff, though you are correct that it will suffice for home rendering.
Put another way, the Ford F150 isn’t a bad truck, and is quite sufficient for most people but sometimes you need a Kenworth T270 box truck to get the job done. People who do heavy-duty stuff for a living (like the people I know) need more than a single HD4850 can offer, and the only way to get that is multiple PCI Express slots; something only the Mac Pro has if you want to stick with Apple.
I guess it all boils down to how you define “just fine”.