I am running an AMD Athlon 3500+ Nvidia 6800GT and it works just fine - key though is RAM - minimum according to E-on for the software is 512 and I started with 1Gig, but found working to be lumpy and rendering slow as well.

Upgrade of an additional 2 Gig and this is a completely different program. 

I asked the same question on Cornupcopia and the answers I got were that RAM is king (if you have a decent system).  From there processor choice is how much can you afford.

There are some people who are using old PC's (P4's etc.) as render cows to speed up the process - the comments are generally good from them.

Hope this helps.