Firstly, Welcome Aboard
Right then, what are the rest of your system specs?
Was it only the graphics card you've changed, and did it run fine before? (With/without GWX2.0)
Is your new card much better than your old one - does it need more from your PSU?
Have you deleted the previous graphics card's drivers?
And finally, for now, what OS are you using? If it's 32-bit XP I think you need Catalyst 10.12 not 10.5 (but I could be wrong, does any other game work?)