Slackware is probably your best bet. Don't know how long since (or if ever) you used Slackware, but it's my favorite and system requirements have not gone up much over the years.
Current minimum CPU is a 486, several gigs of disk space depending on what you install, includes only the basics (GNOME not included anymore), but all packages included are the most recent stable versions.
The only problem in terms of speed I would think would be the need to compile "the extras" (though you can get "SlackBuild" scripts at http://www.slackbuilds.org which makes the whole thing automated) . KDE would definitely be slow, but you can still use XFCE et al., or just the console.
-redblade7
Rogue Central @ coredumpcentral.org