I have a contact who buys off lease Dell Precision workstations, and when I feel the need to upgrade, I buy from him.

My current model is this: http://support.dell.com/support/edocs/systems/ws490/en/ug/geninfo.htm#wp1060065

But, my coworker, who also had this model, built his own i7 machine and its much faster. Two years ago, I paid $450 for mine with 4GB of RAM and 2 Xeon dual core 2.0 Ghz processors. One downside is that it only has one video card slot, so you have to buy one fast video card instead of 2 reasonably fast ones.

You can find these on ebay.