The chipset i850 is the first Pentium 4 chipset and provides suport to Rambus Memory, clocked at 400 Mhz, giving a bandwidth of 3.2 Gb.
The SiS 645 is one of the new pentium 4 chipsets, which suports DDR tecnology, particulaty the new DDR333, giving a bandwidth of 2.7 Gb.
As you can see, the 850 provides more bandwidth and therefore, more performance. On the other side has a higher latency and is more expensive.
I'd personally buy the 645, or maybe the VIA P4X266A (DDR266 -> 2.1 Gb), because in my opinion it has a beter performance/price ratio. But remember, if you want the best performance, i850 is your choice.
Look at Tom's hardware and read some reviews, it may help you.