From what I've seen, the 850 MOBO's were a socket 478 using Rambus memory and AGP 4x video slots.
Socket 478 you should be able to get a processor over 2 Ghz, and the Rambus memory IS faster. You already have 1GB of memory. The 4x AGP slot is a problem you cant overcome, but I'm not sure how MUCH difference it will make. On my old system, switching from AGP 4x to 8x meant the difference in playability. But then your processor is faster than mine was then, and you have a much faster video card than I did then. Maybe yours will work ok. Unless you can find a good deal on a processor though, I'd wait and upgrade the MOBO and Processor at the same time.