I found an older post about this and one person said it's best to just use one computer but a dual cpu system, I don't agree with that.
I'll be running huge samples and a power hungry VSTi and it sems tm me that 2 are better than one.
I don't think any of the networking apps for audio are quite up to snuff yet. The most reliable way to do it, is to get an interface for each computer and use digital audio connections (S/PDIF, ADAT) for I/O between systems. This is like treating each machine as a hardware synth or processor.
In general, you're right... two computers with single processors that are 2.5GHz will provide more power for VI's than one computer with a dual core processor running at 2.5GHz each.
This thread had a lot of info and differing opinions by people who's tried it out. There seems to be an overall consensus about the systems being a real pain in the butt stike. But a few people said they made it work.
I'm curious if any of the quad users (dual dualcore opteron) users have tried gigasampler with their rig. It sounds pretty stable. Check out my Thread "PC Benchmarks and Computers of the Future." has a link to the people who are building them. With those server style boards able to run so many drives with independant RAM and cpu's on each channel, seems ideal - and not too pricey.
I've got a similar setup in my studio except that my main machine is a G5 running Pro Tools. I just go 'old school' - Midi and AES from one to the other.