You don't give your GPU specs. but Google says it's a 32MB ATI Mobility RADEON 7500 video chipset; so you should have enough GPU (graphics) power to run Rad with ease.
Dual boot Ubuntu and XP on such a small system indicates a tendency to pack everything you can into small boxes. With 768meg, there tends to be a lot of virtual memory swapping, which is itself not conducive to 3D type work.. I just have this feeling that your system, though great for many things, may just not be on par with the minimum requirements for Rad.
I may be completely wrong.