I just found a 2008 Mac Pro in my price range. Is this the one you were thinking of?:
Two 2.8GHz Quad-Core Intel Xeon E5462 Processors (8 cores)
Yep. Buy that machine, it'll be great for what you're doing.
(I have that exact one, and I'm typing on it now.)
(Although, no offense, given what I originally paid for it, I'm sad to see the price range it's entered.