1. One major update has been released: SP1. This incudes bugfixes, WMP 9, and an "Add/Remove programs" update ordered by court. You can choose what Windows XP use as default for internet, e-mail, etc.
It's distributed through Windowsupdate.com, Windows XP's built-in "Automatic updates" and new Windows XP CD's which already include SP1.
2. The CPU isn't important. The memory is. I'd say:
P-MMX 200 MHz with 96 MB RAM and 16 MB RAM SVGA. 64 MB RAM will result in a painful overall slowness.
3. At the moment, Samba supports NT Domains, Active Directory, etc. So if Microsoft doesn't take steps against, Linux (in combination with Samba 3) is compatible with Windows networking. I don't know about Mac OS X, but I think they're still at Samba 2, which doesn't do NT Domains very well, and doesn't support Active Directory at all.