dual cards are rubbish better to spend the money on one big card. Please post your motherboard so that we can see what connections there are??? 4gb of ram is a waste you will never use that much I'm thinking of upgrading to 3gb but thats only because I play graphics intensive games on the highest settings and use autocad occasionally. The 7900gt should easily get you that many fps. Your Ram bus speed is abit slow and the pentium 4 is ok for now but it is a bit slow check here
http://tomshardware.co.uk/cpu/charts.html