I agree with what monaarts said, but I just wonder for a second at the pics posted showing a MBP running 2 additional screens (screens that we assume are Thunderbolt ACDs). So I think it's possible that with a 21" iMac and one of those new ACDs, you could daisy chain through the ACD to a non-TB monitor and run 3 that way, but it's speculative.
I'm planning to run my 27" TB iMac with a 30" (over the painfully expensive dual-link DVI-mDP adapter - buy from Monoprice) and also a 720" projector. Since I'll be pushing like 9 million pixels, I opted for the 2GB video card.