The nForce is a computer chipset created by NVIDIA for AMD CPUs. At launch, the nForce offered innovative integrated features not found on other Athlon chipsets of the period, including optional integrated video, dual-channel DDR SDRAM support, and high quality integrated sound based on NVIDIA's own sound chip. The nForce4 is the newest version which features PCI Express.
The nForce chipset was remarkable for several reasons. It introduced dual channel memory to the mainstream market, doubling theoretical throughput, offering excellent performance most especially in workstation class benchmarks. This was deemed necessary largely because NVIDIA anticipated the era of high performance onboard graphics. Fast high bandwidth DDR based main memory was required, to make sure the video hardware was not constantly stalled waiting for the main memory.
If you want to know the difference between nforce 4 ultra and regular nforce chipsets, look here: