With an onboard Intel Graphics Media Decelerator, what do you mean by "I do a fair bit of gaming?" The games most of the people around here play probably wouldn't even load with your graphics, let alone be playable.
If you play just Flash-based games, or 2D titles, almost anything would do. If you want to play more modern, graphically intense games, spending money on upgrades to what is essentially an obsolete system would quickly become wasteful. With a $550-$600 budget, minimal manual dexterity, and the desire to build a system yourself, we can probably help you create a decent gaming computer. If that is the case, please post a thread in the New Build section according to the format in the sticky on how to ask for new build advice. Otherwise, we need more information on what games you are playing. A RAM upgrade to 2GB would probably offer some improvement for you. A dedicated video card would open up a whole new world of games for you, but a powerful one likely needs a new PSU as well. Assuming you have a PCIE slot for it, a HD4650 or HD5570 would run even on your stock PSU (unless it's a Bestec, then maybe not) and be a major improvement.