I have an x61 myself, bought it last year to add to my Thinkpad collection. I haven't done much besides put in some real to verify it actually works. And suggestions on an OS?
The more straight forward (and 64 bit) candidate would be Slackware 15.0 with a few of Alien Bob's slackbuilds.
But, of course, the retro computing approach mentioned by another poster would look really nice and be a conversation piece.
Port any drivers you need with AI.
Only half-serious...
Win2000 or WinXP would be a better choice due to numerous reasons though. Win2K with an SSD is absolutely going to fly on that thing.