Asus and MSI make excellent boards. The P4B533 from Asus would be one of my first choices as the price on them is down ($115 at newegg). If you want 8X capability - the P4S8X is running $135 at newegg.
For MSI boards, the 645E Max2-LRU is a pretty nice board for $112 at newegg and the 845GE Max-L $95 is also a pretty good deal. I all depends on what you want and what your budget is.
I hope that helped.
Oh - and welcome to the board Nixon!!
Which reminds me why I want to start working on a beowulf-on-a-backplane design: I want to be the one to write the program called "clusterfsck."
-- Adam J. Thornton, a.s.r.