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?
Windows 98SE. 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.