I started to do a little research on your system. It appears you have a PCIe x16 video card slot available and DDR2 533 RAM with a 300W power supply. I was trying to get specs on your power supply as most of he better video cards these days are going to need 350 watts (Nvidia 9500 GT) or more. Based on your CPU I wouldn't go over a 9600 GT which needs 425 watts.
As close as I can tell you've got a uATX/BTX form factor PSU but I'm not convinced of that yet so would need to do more research.
Anyway, your options are to get a lower end video card (Nvidia 8600 or 9400 or lower) which will give you a marginal performance boost or get a new PSU to power a better card. It would be good if you could measure your power supply and post the dimensions here.
As to RAM it looks like you have 1 Gb. I'd get 2 new 1 Gb matched sticks of DDR2 533 or better. That alone will give you a nice bump up in performance.