I have done some thinking, and realize that I don't have the required docs to interface an FPGA with a PIII correctly. So I had an idea: if I really can't use the PIII, I'll just save it for later and implement my own CPU on a separate FPGA, or I could use one FPGA and make some sort of SoC. That way, I can work with a familiar architecture and an instruction set I know, because I made them. But I'll try my best to use the PIII. Perhaps I could strike a deal with Intel. Or maybe I could use a PowerPC cpu, or even an ARM core.

As for the RAM, having it be socketed would not give the required memory bandwidth I need. So the RAM will instead be 512MB of DDR400 RAM, split across 8 32bit chips. This will give a bandwidth of 12.8 GB/s. That'll be able to handle a GPU and CPU at the same time no problem.