I had an Atari 800, with extra memory. Besides the stock basic I wrote some programs in an extended basic, C, and even tried lisp and forth. Even a little assembler code, though that was a royal pain and not as easy as IBM mainframe assembler. It was a pretty good computer for back then, but it was somewhat impaired by confusion with the game console. It did run some pretty good games for the time, though.
I kind of doubt that any of the old 8-bit computers are worth anything. Maybe one of the old Altairs that didn't do much but light up led's?