Simple Nvidia card question

I have a p4 2.0 512 Gforce3 computer and would like to just upgrade the video card for now til I can afford a top end system. Should I get teh 5600 256 ram or will there be much of a difference if I get the 5900ultra 256ram card? I really cant see spending an extra 300 bucks if my CPU is restricting a higher card from preforming and might only get a few more frames per second. However if the deffernece would be waaay better graphics and at least 10 more frames per second then I could see getting the top end card.Kilstorm