Dual Video Card Vs. Single for dual screen gaming performance?

I am currently trying to run dual monitors from my ATI Radeon 3400 and I see significant quality restrictions while gaming. I primarily run Starcraft 2 and would like to try running with dual screen at the same quality or better as when I only had 1. There is an Nvidia GeForce 8600 GT available to me and I was wondering if running my second monitor from one would increase the gaming performance of the other. Also, which card would be considered "better" than the other?



My Dell Inspiron has 4gb ram and a 3+ GHZ dual core processor. My main screen is full 1080 with HDMI (no cable now though, just the DVI)



Please advise, thank you!|||There's no such thing as an ATi Radeon HD 3400. My guess is that you have a Radeon HD 3450. There's no comparison, the 8600 GT is far more powerful. I would just use it by itself and keep the HD 3450 as a backup card.|||Single is better