hiya,
I have played AH with a 300mhz machine that had 96M ram and an 8M ATI P.O.S. video card. I had FPS in the 15-18 range in normal flight.
So, it is my opinion that you could do some tweeking to get the FPS up. Also, the GeForce MX cards are really cheap and will help your frame rates greatly or at least get them up to the point the the CPU is the bottleneck.
Also, you should post you question over in the H & S forum as well. There are a lot of folks there that can truly help you out.
F.