Acorn Atom, back in 1980 or '81 (.5KB RAM, .5KB video RAm though we added extra to take it up to 5/6, and a colour board offering two four-colour palettes (that never worked well) and a maths chip). Then a gap until an Atari ST (520STFM, but ended up with 4MB of RAM and various external drives) followed by five PCs to date.

Aha, it's on the same site: http://www.old-computers.com/museum/computer.asp?st=1&c=80