The dreamcast scene is just a bit faster than the gp in terms of emulation. Only the dreamcast has neogeo cd emulation. And even sega saturn I heard somewhere :S, but I've not tried that yet.
There's an illegal megadrive emulator that works full speed fine, but some people complain about sound problems. I've never had trouble with it though.
Basically, all 8bit is full speed. 16 bit is getting there and there are some other interesting emulators. If you want a bit better emulation than the gp on your tv, then the dreamcast is what you should get.
I remember you were looking into a chipped xbox a while ago, as was I. But I couldn't afford one (and it would be a bit pointless as I have a ps2 anyway), so I got a dc (which it looks like you're doing the same).
But I hardly ever use the emulators on the dreamcast now, as the dreamcast is a perfect dreamcast emulator
. Boots cds without a chip so if you feel like buying a load of games, then downloading them so it's nice and legal. Then the backups will work fine.
Dreamcast has some kick ass games
.