With the two video cards and the 4GB ram a 64bit OS be it XP, Vista or Windows 7 will make use of all the system memory you have therefore improving overall system performance. When calculating memory Video memory is also factored in to the System Ram when using 32bit OS's. Will this improve your overall AH game play? Hard to say. What do you have your current settings set at? Is everything maxed out and are you getting your max monitor refresh rate? To many variances to say "YES" this will improve AH. I am not familiar with prebuilt systems and the motherboards they use, chipsets, what BIOS settings they have limited or deleted from the BIOS makers original BIOS. Conclusion, overall system performance will increase with a 64bit OS.
TD