I read the post thread and it seems you're a good example of trying to keep a "gamer" happy. You will find that kicking and screaming your PC into a happy game realm won't be as easy as it appears. I offer this because, for one, you have an OEM type PC, probable HP. But, you didn't state what model# you have because OEM based systems can be troublesome to really get game ready. If not, read on...
Next, I would concentrate on video(its ram+chipset) and power which already some posters offered, at least PSU. The video side has always been keen to provide the best gaming experience and the PSU along with it to support the new power demands it may require. I always post that cooling is a prime consideration as well in order to sustain power and video outputs at high levels in order to keep all working. Looking into those three areas should make your gaming experience a good one. That's video, PSU and cooling and then ram if needed, because 4gb is often enough. The next level is making sure all this is certified or rated for "top shelf" or best offering, like PSU is 80+ and ram is SLI rated or similar. That ram is heat thermo-alloy shielded, etc.. Otherwise, sooner later something will falter because either you do prolong play or find the weakest link. I have found larger PC cases a must for best air circulation. Alas, was this desktop offered as a gaming one?
tada -----Willy 