The nForce is a chipset that is being developed by nVidia for running AMD based systems, along side with DDR Ram. Just like how the i815 and KT133A are chipsets, the nForce is a chipset as well.
Specs thus far say it'll come with an onbaord GF2MX, onboard 5.1 Audio, onboard 10/100 NIC, and the usual. It'll also run a new memory accessing system. Which should increase performance.
--Mark