I actually was having a similar issue myself. Here are some benchmarks.
AMD Radeon HD 6770M - Notebookcheck.net Tech
Larger list here:
Mobile Graphics Cards - Benchmark List - Notebookcheck.net Tech
My advice: I would honestly just go with the 21.5 in iMac unless you want the 27 in screen, in which case I suggest bumping up to the 1999 model that has the 6970m in it.
I just bought the 1999 model yesterday, and I can honestly say I spent about 2 weeks looking at every aspect of all the different iMacs.
Here's what I've basically come away with. The 6770m probably could do it at max settings, but probably not at native res. The real thing bogging the card down is the RAM, its only 512mb. Most PC's packing this card come with 1gb RAM. And recommended specs, according to the SC2 auto settings are that it should only be rendering high instead of ultra quality textures if theres only 512mb RAM on the card.
I haven't had a chance to install SC2 on it yet, but I likely will install it today and see how it goes. In the mean time I would poke around these forums and google/youtube, there's a lot out there on the gaming capabilities of these things.