I asked the same question a year ago

http://www.dual-boxing.com/showthread.php?t=10713

Summary:
I ended up with an MSI Wind. 1.6GHz Intel Atom, 1g DDR2 667MHz (upgraded to 2g,) Intel GMA graphics.

I get ~8-12fps in IF, 6-10 in shat, and 20-30 most other places. One interesting thing to note is that putting all my settings on max didn't affect performance at all, the bottleneck isn't the GPU or RAM, it must be the CPU.