Forget the mobo and cpu, your entire gaming performance with a mid/high range GPU is what matters (and your PSU). You don't need another board or CPU. The
Q8200 will game around the same as the pentium 'sandys'. The G870 or similar. It will be absolutely fine for anything up to a 670 or 7870. After that I would simply get the best CPU for your socket, not a whole bloody mobo, windows installation, CPU - it will probably at best give you 10-20FPS increase. Better spending the coin and getting an SSD and Graphics card like the 670.
Spend:
GTX670 graphics card
256GB SSD (and install 64bit Windows 7 on it - make your computers super fast and also make available the other 4GB of RAM you have installed)
Corsair 750W power supply unit
Overclock the Q8200 to 3.0GHz (easy)
Play any game in high definition at 60FPS.
Budget less than 700.
BUT!
Please
tell us what PSU you have exactly. Model number please.