Here:
P4 3.0 ghz
2 gb ram
Nvidia 7600gs 256mb AGP 1024 x 768
Win XP SP3
I run BS pegged at 20 fps and have no problems.
I do have some details turned down of course, but it still looks good. Having several KA-50's on screen will slow things, and multiple ground-based rockets launching close by are a no-no. I think CPU is more important than GPU with BS, and there is a tweak for Vista owners with dual core that can improve FPS.