If it is only a 128 mg card, I will flat guarantee that it is worth the risk... Try to plug in to local gamers, or a local back-alley tech shop, where used or known-good cards are easy to come by... Or go to your local Best Buy (*spit*) as you can return it if it doesn't work... And if you are having trouble with ATI, then try to find an off-brand that uses an NVidia chip instead - That gets you out of ATI altogether.
The one I have in my test bench right now is an off-brand NVidia from Best Buy (*ptooey*)... It is a 2g and cost about 35 bucks...It was the third one I tried for a client (who wound up buying a new machine anyway)... So I know about the hit-and-miss nature of such things sometimes...
But in your case, jumping from 128mg to 2g vid will be very substantial, and will probably eclipse anything you have done so far... even if you have to pay someone to handle it for you. I doubt that will be the case, since new cards will probably work better (driver-wise) with Win6 than with Win5 - I bet the OS upgrade will help with the vid issues.
So I looked a lot but finally gave up. I do not have much of any real problem as long as i restart FF to get some ram back sometimes due to 80+- plus leaking tabs, or the cpu load is too high with pages like http://www.zdnet.com/how-the-new-windows-8-license-terms-affect-you_p2-7000003028/.
And regarding that link, i would have to get clearer confirmation that i can transfer W/8 to a new mobo, without upgrading. But for now it is manageable. Hey, i used W/98se and dial up until about 2005! Let me manage the nation debt and i think gov. would shrink about 50%.