If you upgraded any more than a gt240 you would get a bottleneck. If you really want a strong card you will need more CPU RAM and PSU.
I don't want a stronger card/cpu. I want to max out my old PC and use it until I can build a proper gaming PC
Any suggestions on RAM? 4x1GB, 2x2GB 1x1GB? 1x1GB+512x512MB?