TBH, it sounds like the artists want shiny new toys to show off to their colleagues at their employers vast expense. I'd be inclined to review whether the outlay will provide any kind of productivity improvement. And even at that, as prior meantioned, the Core i5 iMacs should be plenty powerful enough, and I'd really consider a standard PC since CS4 works more or less the same on both Windows and OS X.
Not a good time for splashing out on toys, atm.

