Do you have the money to buy a new motherboard and SLI two graphics cards?
The 7800 GS is a pretty good card, but it's not good enough. It's about X850 XT PE range but with newer technology. If I were you, just buy a X850 XT PE. Even if it lacks Shader Model 3.0, it's relatively cheap price and power brings it above the 7800 GS. Sorry, for the nVIDIA fans, but ATI is still the King of the AGP market. The 7800 GS is about $100 more than the X850 XT PE, but it doesn't beat it outright. Instead, it stays at about the X850's range. Get a X850 XT PE. It's the best AGP card for the money and for it's performance.