the 27" versions of the new iMac come with either the AMD HD 6770M or HD 6970M GPU. How do they compare performancewise? Is it worth its money?
It seems the answer is 'not huge, but noticeable', maybe 15-20%. I'm not sure what the price tag difference is off hand, but if you're working on graphics apps, games, video - get what you can afford, with the iMac's not being upgradeable in that sense, what can it hurt to do so? With an all in one I'm a big believe in buying what you can afford to get.
That said - if you're going to replace the machine next year? Don't worry so much.
You can compare these graphic cards at Tom's Hardware