Core II 6700 working really well for me. A great, cool running processor.
I think AC11 is a heavy processor user. The graphics card would be better with more ram but if you can up the processor it would have the most effect. I have been overclocking and am now running at 3.3 ghz. The only thing that slows things down is a file with excessive s.e.o.'s with many complicated wall profiles and complicated 3d mesh.
XP 64 bit lets you use 4 gigs of ram... Price of ram seems to make 4G reasonable.
And it is really dependent on the scale of work you do.
Good luck.