QUOTE - Cheers
"In order of priority of important features: CPU, memory, graphics card and then 64bit hardware compatability (64bit is coming and it is probably best to make the jump now, and most native 32bit software will run on it)."

What Cheers said!

With emphasis on the CPU. SMP (dual core, dual cpu) is the way to go. More cores=more threads=Much Much happiness...& way faster render times.
What a Rendercow does is merely distrubute the job among many CPUs. So..already having multiple CPUs in you rig is obviously a powerfull start.
Most graphics applications are multithreaded, which means they (should) really know how to take advantage of the extra CPU power.

It's a shame stupidity isn't painful.