use a non-ancient filesystem that doesn't slow down with more than a few thousand files in a directory. I have 22000 emails in one imap folder on my server (and tens of thousands on other folders) and have zero slowdown with reiserfs.
Sounds like you have had good luck with reiserfs? I'm not too familiar with that one... Any preference for reiserfs over XFS? The boxes we run are Redhat Linux boxes, which is the reason I had looked at XFS -- it is well supported on Linux.
It is very well supported under Linux, as far as I know it only runs on Linux - www.namesys.com, Lindows, SUSE and DARPA all support the development of it.
I've never played with XFS myself (Slackware's default install options are ext3 or reiserfs) but anything is going to be a huge improvement over ext2/3.