i think its the i845D chip you're talking about...
anyways....id go woth the sis645 chip cuz it has PC2700 ram support (specifically asus' P4S333) and has a relatively good price.
the i845D supposedly has the best stability of the three...but is the most expensive...
the P4X266 has too many legal mumbo jumbo and performs as well as the sis645 chip w/ PC2100 ram.
<A HREF="http://gamershq.madonion.com/compare2k1.shtml?2096468" target="_new">P4 + SDRAM</A> = <b>BAD</b>