Oh, don't get me wrong! I'm not looking to replace my soon-to-be Dreamcast setup with an X-BOX, I'm thinking about supplementing my setup with an X-BOX. It turns out that my friend broke his X-BOX (by trying to solder in a modchip), and I'm buying it for $35. If I can get it to work again (there is a good chance) then I'll have two X-BOX's, so why not throw one of them into the cabinet?
You definitely can't replace the Dreamcast though, it has classics like Soul Calibur, Chu Chu Rocket, etc, etc...
The only advance on the X-BOX is having a hard drive (eg. no disc swapping) and of course better graphics.