There are some dual Opteron motherboards available through Newegg, but for $260+ I would have doubled the cost of the upgrade. The X1950XT (AGP variety) gives about the performance I’d get from the card if it was PCI-e; anything faster may be bottlenecked by the processor.
My next upgrade will be significant, and that’s why it will be expensive. For now the X1950XT works well enough at 1680×1050.