If you wanted an add-in graphics card, I'm not sure what to recommend. Your 300W PSU is supposed to be adequate for cards that use the nVidia GTS 240 graphics chip. Such cards don't need an auxiliary PCI-E power connection. Cards based on the ATI 4670 or 5670 chipset supposedly want 400W PSUs, even though they, like the GT 240, draw all their power through the PCI-E slot (75W maximum). The 5670 is a DX11 card, but I'm not sure that makes it more future-proof than the others. My favorite resource for checking out graphics cards is the power search option at
Newegg.com - Computer Parts, PC Components, Laptop Computers, LED LCD TV, Digital Cameras and more!. I tend to believe that the ATI cards give the best performance for the price, but I prefer nVidia's drivers. (This is all subjective; I haven't studied it.)