just to note, any 64bit distro would be fine, just do a minimal install then only add in the exact packages you want. I am not 100% sure, but I think if you did a stage 3 install of Gentoo and then emerged kde-base, you'd be well under 1gig of usage. You could then go thru and hand pick the apps you need to fill up the rest.
I'm not sure how Ubuntu/Kubuntu/Xubuntu installs are, but I'd assume they leave you with a fairly large install with tons of extra junk you don't need.