According to industry tipsters, Apple is planning on releasing an updated iMac next week.  Other than that details are somewhat sketchy, but here's what we do know:

"Apple is known to be developing a Penryn-based refresh of both the iMac and its Mac mini cousin for the spring, delivering the cooler-running and faster processors to systems that haven't been updated since August of 2007."

We wouldn't be surprised if Apple also included a higher end graphics option for the high end models.