Is that your PC?
Well, i think it will be wise to save your money to build a whole new system.
And it's DDR, not DDR2 on memory side. Any card recently will bottleneck by your cpu and ram. It doesn't mention about PSU, i assume it has a low PSU, so generally you must upgrade all of your components...
But, for temporary, getting HD4670 will suffice, at least with 4670 you will see performance increase from X800.