I would go with one of the "big ones" i.e. RedHat, FreeBSD, Suse, Debian and not one of the smaller distros though (even if they are based on the larger ones). As far as long term maintenance goes there would be less documenation online for nuanced differences between the niche's and the "big ones", and its easier to find volunteers that are really familiar with the bigger distros. As far as drivers and ease of install goes, as long as the hardware raid controller and Nic are supported by the distro you are set (we aren't installing a desktop here). What's the hardware raid controller for the server?
I would vote for Debian Sarge since it is very stable now, easy to maintain, and I simply like the community nature of the distro. Sarge can take the 2.6 kernel without difficulty (the upgrade for me from 2.4 to 2.6 was simply: apt get kernel-image...), although Debain Stable with backports would make sense too. 2.6 has been noticeably faster on my servers than 2.4 (although they do things other than just serve openacs sites, so i don't know if the differences are found in the other tasks).
