Depends on how powerful you want to go, as well as how much you're wanting to spend. Mac Pro 1,1's (2x Dual Core Xeon CPUs) at either 2.0, 2.66GHz or the Mac Pro 2,1 models which are 2x Quad Core Xeon CPU's 3.06GHz for a total of 8 cores. It's said by Apple that the 1,1 models only support 16GB RAM but in truth they support up to 32GB, same as the 2,1 8-core models. If you can afford the 8-core model, is definitely a faster machine, question is do you require that much power? Will the apps you run take advantage of the additional CPU cores? If so, the 8 core model will do you much better, if not, than 8-cores are wasted. The 8-core can be had for appx. $2,300 or less on eBay, and the Quad core (Mac Pro 1,1) can be had for considerably less than $2,000 on eBay, appx. 1,500. Both are excellent machines with LOTS of life in them. I upgraded from a PowerMac G5 and have been extremely happy.
The only "downside" of the 1,1 or 2,1 models is that they will NOT boot a 64-bit KERNEL, however, they WILL fully run 64-bit applications. This is due to the 32-bit EFI in these models, but I don't believe that is a dealbreaker for you.