I've been playing the demo on XP and have had several instances where the game just freezes. I tried the recent patch but it hasn't effected the problem.

it is very difficult to say what exactly you need or what exactly VideoCard does fit.
First, to run MA it is enought RivaTNT2 with 32MB.
To play MA Gf2-MX400 or Radeon 7000 series will be better
For good graphix and performance better to use GF3.
For good graphix and performance in other new games beter to use GF4
For good graphix and performance in future games it seems that GF5950 will be good choose.

But some motherbords can conflict with some video card (restarting computer when you are playing game for example). So you have to get Video Card with money back warranty.

From my point of view better to use GF3 or (GF4 not MX-440!!!)

If you have the latest drivers the problem usually is that your motherbord and graphix card does not fit well to each other well (e.g. MB have only AGP 4x when the Video Card is AGP 8x.)