Nvidia is working hard on Hybrid SLI whereby you can combine the power of chipset and discrete graphics. Theoretically, it should help you run your games faster but it looks like it will be able to boost your games by 15 to 30 percent.
Nvidia even has a name for its new chipset. MCP78PV will be commercially known as Geforce 8200 and naturally, it will pair with 8400 and 8500 cards to give you more performance.
The new chipset comes in Q1 2008 and it looks like it might even be ready for early February. Naturally if you combine the power of 8200 graphics and 8800 GT you won't get much of a performance boost, but you might save some power as the Hybrid SLI can shut discrete graphics off and let your Windows work on chipset graphics only.