For sheer processing power the iMac Intel Core i7 2600 will beat most if not all the quads and get pretty close to the 6 core. Even the low end 12 Cores will have similar performance.
That's a lot of bang for buck, I'm guessing you use audio programs, so do I (AbletonLive, Logic Pro 9, Numerology, Maschine). I have a 2008 MacPro and love it, but the new iMacs are looking very promising.
Imagine this setup:
iMac with SSD
Firewire/USB soundcard
UAD Satellite
Thunderbolt Storage solution
There's no reason to have sample and recording drives separate on Thunderbolt (There's not much point anyway unless you're multi-tracking a massive band), audio doesn't take up that much bandwidth.
Logic, Cubase, ableton Live will eat this thing up, it will be beautiful.
If you need internal cards, you'll buy a MacPro in the future otherwise, the iMac is looking like a great option, especially for audio.