I have just upgraded my iMac to the new 24" 3.06Ghz because my previous one struggled with some of the more Amazing things that Omnisphere can do.
I'm very happy with it so far, I have opened up previous Logic projects where I had to freeze the tracks to be able to play them, unfrozen everything, and I just can't max out the processor. Nice.
I'm running a second 24" screen so I can have a big mixer window in one and an arrangement window in the other and it is handling this fine.
So my advice is buy the iMac and save some money.
All the Best