Writing for old systems _is_ cool; its fairly hardcore, in that few peopel do it, though the Atari 2600 coding scene is still pretty alive.
Its not too hard to get assemblers and kits with ready to use routines for NES etc, etc, too.
Depends what you want to do though.. ie: SNES is pretty well understood and pretty powerful, but 3MHz CPU or somesuch. TG16 had some cool features in the CPU, but it can be difficult to code for and less general coding support outside of Japan. GB and such, easy.. very limited hardware, and well documented.
ST and Amiga have full gcc environments.. hell, you can run Linux on them
(No, Iv'e not tried running Linux on the gp2x inside of the emus. Perv.)
So lots of options.
Also arcade machines; I started a game that wads meant to run in Space Duel vector hardware once...
jeff