Hey-- I agree it is your weak point, but your link doesnt work.
What kind of gaming performance are you looking for, and at what screen resolution?
If you are looking for a small performance boost--the best low profile card you could put in there is most likely an ATI Radeon 5570.
Otherwise, really your only option is to move your components to a full size case so that you can fit a full size graphics card and the improved power supply you would need to run it. Inexpensive cases are $20-40, and a good medium range power supply would run you $40.-- However this would let you upgrade into a pretty decent performance gaming machine.