This might help you make a decision (page 7 I think): http://www.adobe.com/content/dam/Adobe/en/products/creativesuite/production/cs6/pdfs/adobe -hardware-performance-whitepap…
I use a NVIDIA K2000 and that works a treat.
Here is an old post you might want to look at: Re: Which NVIDIA graphics card for CS4 & CS5.5 Premiere?
To be honest I would suggest upgrading. When I investigated about getting a new graphics card I had to work out how much of a budget I had and got the best I could.
A great research site GPU Database | techPowerUp
Thanks for your help! Appreciated!