The 680 chipset has enhancements (automatic overclocking of the bus) in conjunction with a select few nVidia display cards. I don't think the 8600GTS is amongst them but the 8800 (GTX, maybe the GTS too?) is.
If I wasn't so knackered, I'd re-find the link on the nVidia website which clarifies this.
If I wasn't so knackered, I'd re-find the link on the nVidia website which clarifies this.