I went from a VIA Kt400 to NF2 and saw no improvement on a 32 BIT system. The big claim to fame for NF in 32 BIT systems was dual channel memory but I never saw any significant improvements with my Barton CPU even on a memory only benchmark.
I haven't gone 64 BIT yet so I don't know personally about the NF3 and NF4 performance vs the VIA and SIS. But when the NF3 and Kt800 first came out the KT800 was the prefered chip. That has since changed with chipset updates.
The benchmarks I've seen have shown no significant difference in performance between VIA and NFx. The SIS are usually a little slower. I seriously question performance going "way up" on any chipset change, especially on 64 BIT AMD where the CPU and Memory talk directly and the chipset just handles I/O.
The other thing they did not mention above is that an SIS MB is a lot cheaper than the VIA or NFx chips. So if your building systems for resale SIS is the way to go.
The other differences will be in number of USB/firewire ports, LAN type and quantity, support for dual video cards, etc.
Thinking about it. The reason I stopped running benchmarks on my own system was that I never saw any substantial improvements for any of the "hot new chipsets and features".
NF3 and 4 are 64 BIT. The NF3 is AGP and NF4 is SLI.