Hands down the DFI Ultra Infinity with the 12/18/03 Beta Bios for BH5 sticks.
I just fired up a DFI UI (my second system with one) with an XP2600 Mobile a couple of hours ago.
I am at 254x10 on air running 11,3,3,2,2 @ 3.300 Vdimm with 2x256 Corsair XMS PC3200 ver 1.1's
with Commands per Clock...