If I want to buy some cheap computers and set up my own little renderfarm, what should I be looking for?  Is processing speed the most important thing, or ram, or both?  Any advice appreciated.  :)

Generally the faster the better - thats quite obvious. The amount of Ram depends on your scene complexicity. I'd recommend 1 GB of ram minimum, better would be 2 Gig for these huge scenes. Depends a bit on what you expect and want to spend.

But Ram and processor are the most important components, thats for sure.

Processing speed is always going to be the answer as long as the scene your rendering doesn't exceed your memory, there is a render farm already set-up for Vue and if you work out the maths it would cheaper and faster to use that service.

I have two computers, one a cheap laptop, the other a really nice desktop, and they both seem to render at he same speed. I think that you might look into 64 bit prossesors if you want somthing to go fast. but yes, ram is important too