From XP 32-bit(2.5G RAM) to Vista Ultimate (8G RAM), preformance decrease!!

Here is the preface: I know little to nothing about the technical aspect of computers, please refrain from laughing :[

The title says it all, really. I bit the bullet and bought Vista Ultimate today, which "unlocked" 5.5G of RAM, yet when I quad-box I see no preformance increase what-so-ever, maybe even a slight decrease.

I am EXTREMELY annoyed, seeing as I bought Vista Ultimate, 8G of RAM, and a quad-core, yet I still run 20 fps on my main when quad-boxing.

Is there something I am doing wrong? My system specs are:

Intel(R) Core(TM)2 Quad CPU Q6700 @ 2.66GHz 2.67 GHz | 8.00 GB of RAM | Vista Ultimate.

Also, I have (2x) NVIDIA GeForce 8800 GTX.

Please, if anyone has advice, share :[ I've searched and found people with similar problems, but the answers are almost always too complicated, or too basic.

Is there anything I need to disable, and how do I get to it? Anything I need to update, and how do I do so? Please, help me out!