I have:
XP 32 bit SP3
2 GB of RAM
AMD FX57 at 2.8 GHz
NVIDIA 8800GTX 768MB

Not too shabby but anything but not exactkly state-of-the-art anymore. Game still set everything to ULTRA for me with 4AA 16AF. Actually worked great in both battles with just a tad of lag when zoomed in close during the landbattle with a lot of units. I'll set it down a little though to get that extra smoothness. Looks like it is more GFX than CPU dependant since my GFX card is a lot nicer than my CPU for being 2009?