The Mac Pro 3.33 GHz 6-core is the best bang for the buck in a Mac Pro. B&H Photo has the Mac Pro 3.33 GHz 6-core in stock with free shipping (and no sales tax outside NY), and you can also save about $75 on AppleCare at B&H versus what Apple charges.
See my recommended Macs page for direct clickable links.
You can have this all done for you as an MPG Pro Workstation. David S did, and writes:
I just saw your recent post on MPG about the 6-core 3.33 GHz machine tricked out with the Accelsior PCIe SSD for boot drive. Well, I am absolutely thrilled with my machine. I process RAW files from my D800 using Lightroom and Photoshop CS6 and it is very fast. Sure beats my old MacBook Pro. The monitor is great and I love having the old Cinema display to hold all the PS tools.
I am really enjoying the DAP and Making Sharp Images subscriptions, too. I've learned a lot. Now, if the finances allow, I am likely to purchase a much better chunk of glass. Or two. :-) The D800 is a truly transformative camera.
13" MacBook Pro
Get the just prior model 13" 2.8 GHz MacBook Pro with extras for $1199— free shipping, free software, an almost giveaway Canon printer, $50 off Adobe Lightroom 4, 50% off AppleCare (this model does not have USB3 or the newer graphics system, but it has a fast CPU).