The Dreamcast is an excellent machine. It runs games great and is fairly hackable.
However, its not very likely that it would be able to run UAE at an acceptable speed. The main reason for this is that the Dreamcast is basically just a fancy 3d rendering machine. A lot of emulation requires raw cpu power, something that the Dreamcast has little of. This is why emulators on the Dreamcast run slow. Sure, it has great hardware rendering and decent sound, but it's NOT a PC. Same with the ps2 (although I think the ps2 might be a bit more powerful in this department).
The xbox is a more likely choice since it's basically an 800mhz PC.
