The first question is: What is your budget?
Unless you can find a great price on an 850XTPE, you'd be better getting a new mobo + video card if you want to make the jump to PCIe. There are mobos that support PCIe and s754. Take a look at the Soltek SL-NF4-754RL, Abit NV8 NF4-4X or Gigabyte GA-K8NE. All of these boards are around $75. Put it with a 7800GT and you're smokin'
Back to the original....What's your budget and wha tdo you have right now?