if you go DDR, go SIS 645 - it performs slightly better than the i845 and certainly costs less...the board is rock solid as well and most 645 boards are awesome overclockers.
if you go Rambus, well, there's only one option - i850...at higher clock speeds, the i850 is faster than the SIS 645 when running on equivalent FSB speeds....still not worth the price premium if you ask me (board prices of course, not rambus vs. DDR), unless you plan on takikng your chip well over 2.2ghz.
check out the MSI-6533 - good board.