Your processor isn't terrible but it also isn't very good at all. Even a $50 Athlon II x2 is nearly twice as fast so yes, upgrading would be a good idea although it isn't absolutely necessary quite yet. Considering you are using 7600GSs I would guess your motherboard is 939 which means it would need to be replaced for a CPU upgrade, and probably your ram as well.
All of this doesn't have to be that expensive however. Basically you need to decide how much you want to spend. You can go with an Athlon II x2 for $50-60, an Athlon II x3 for $70-75, an Athlon II x4 for $100 or a Phenom II x4 for $140-160.
A reasonable motherboard for any of those can be had for between $50-100. They come in 2 varieties, AM2+ which uses DDR2 ram or AM3 which uses DDR3. 2 gigs of DDR2 starts at $40, for DDR3 $50. 4 gigs is optimal but 2 will get you by. The rest of the components like the case, hard/dvd drives and PSU should be recyclable from your old system although you may want to upgrade the PSU depending on what video card you end up going with.
As for the video card, yes, a single 9800GT should be much better than your SLIed 7600GSs. The 7600GS was one of the weakest 7000 series cards and if I had to estimate the 9800GT would be over twice as powerful as your SLI setup. If you don't have the 9800GT already I would recommend an HD5670 instead. It performs similarly but is DX11 compatible and much more power efficient. Also most current AMD motherboards are crossfire and not SLI compatible. If you can afford more I would recommend an HD5770 instead which a nice card at around $150 and performs well at resolutions up to and including 1920x1080.