One man's "large project" is another man's everyday job. Without knowing more details, nobody will be able to give more specific advise. It's the same as with hall those discussions on performance, MP rendering, CUDA/ OpenGL that we have a-plenty here on teh forum every week... In any case, since your options with an iMac are limited, anyway, it's probably a moot point. Stuff it with the maximum possible configuration and then see if it works for you.
I love Apple products. I have many of them in my home.
I don't use any of them for video work however. You can get a lot more bang for your buck with a Windows machine these days. Seriously. Go price out those same specs with HP or somebody similar. Windows 7 is a very stable, very useable operating system. (Don't bother with 8.)
Also, if you want a computer that will last you a while, I prefer to get a workstation or desktop kind of computer vs. a laptop or an all-in-one because it's easier to service and update if needed. Try opening up an iMac and swapping out the graphics card...
For video at work, we use HP and Levono workstations. For video at home, I use an HP desktop.