I was under the impression that the 4670 fell between the GT 120 and GT 130.
I don't know about the notebookcheck charts. As far as I know, the GT 130M and 8800M GTS are practically the same card, and yet they're separated by about 10 cards on the notebookcheck charts.
We'll have to wait for the iMac to be benchmarked, but I expect the GT 130 from the previous generation iMac to beat the 4670, which means the 4670 will make a POOR card for anyone but a casual gamer at the 2560x1440 resolution of the 27" iMac.
For someone looking to game, I don't think there's a better deal on the market right now than the refurb 8800GS iMac. Personally, if I can justify the purchase, I'll be waiting for the 27" Core i5 4850 iMac to hit the market, although I'm sure I'll be waiting until at least January.