developing resouces for the dreamcast are a bit of a mess at the moment, i try to get all the tools at one place (my site dcenews.co.uk) but having said that the kos development libary is very advanced (some say its better than the sega one in places). Even now there is suppport for the BBA and LAN adaptors and the modem should be supported. One guy is working on an 8 player online rpg for the dc (also pc,mac,linux)
its a shame that so much of xbox homebrew uses the offical SDK instead od the open SDk
I would also recomend SDL http://www.libsdl.org/index.php as is cross platform ( dc, psp, pc, mac, ds, gp32/gp2x to, name a few)