I have a neat idea...how about you upgrade the monitor to a 20incher and the HDD to a 150Gb Raptor. These will make a better gaming experience. You should upgrade the video card to one with DX10 mid next year.
The monitor is on its way. No I have to decide which upgrade is next. I am thinking the case is next. (I have been needing to get a new case for years, always putting it off).