That might be the chipset's backbone transfer bus
AMD/Nvidia up the ante with "Hypertransport" - currently 800Mb/s but capable of more.
The purpose of ultra high speed inter-chipset busses, is to reduce congestion during busmastering or onboard graphics use - the dualbus feature also helps.
Most machines in a group test that had nForce chipsets, did NOT use the onboard graphics - but had a more powerful AGP card.
Talked to someone today, who said:
Intel's run cooler, are more stable, and perform better in multitasking - Corporate users still favour Intel by a considerable margin (naturally, like they favour IBM, Compaq, and other big name, expensive INTEL builders with grade 1 support)