The fastest cpu you can get :) I have a 1.3 ghz p4 and my last render took days. More ram will help with swapping to the HD while rendering. My last image ate all of my 512 mb of ram and was still using virtual memory. the task manager reported 634 mb ram being used . I allocate 10 gig for virtual memory. Lots of bump, reflections and transparency will slow you down.