the x300 is designed for gaming but not intended to actually be used for gaming . i would look at buying an X1600 in your budget range. why not try and get a second hard card? you would be able to get a better card for the same price if not less.
did you try turning the settings down on the demo? make absolutely certain that AA (Anti-Aliasing) and AF (Anisitropic Filtering) are disabled or you will get nowhere with an X300