I think only edge nVidia's drivers have is in the SLI configuration where they seems to have better performance, but that is about it.
HP Link for your PC
First problem I see with it is the crappy 250W PSU, which you will need to replace if you want to have any decent graphic card.
I would suggest you to get this HD 6850, it will cost you 135$ after mail-in rebate:http://www.newegg.com/Product/Product.aspx?Item=N82E16814161384
And this Antec PSU to go with it will cost you 70$ + shipping cost:http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035
Lastly no, your motherboard does not support CF or SLI setup.