This has been hitting the rounds, and it's a great addition to everyone's Xbox 360 library - the Dreamcast version of Soul Calibur on Xbox Live Arcade.
The XBLA version will boast "hi-rez" graphics, which is a common attempt to make the games look more contemporary. I don't know if that will work with the Dreamcast faithful. After all, Soul Calibur looks pretty great as is. I've also noticed that upping the resolution doesn't work for older games. True, the polygon models are sharper and cleaner, but texture maps - details like floors and clothing, for example - tend to look more blurred.
My mind races to the Saturn emulators that were surfing around the internet a while back, and playing NiGHTS in hi-rez. To be perfectly honest, I preferred the original Sega Saturn look. I like that heavy brushstroke style, that boxy cubist look to Saturn graphics. It's what gives its games their character. On my PC, NiGHTS didn't really look all that hot.
My experience with the Project 64 emulator, which plays Nintendo 64, has been very similar. Then again, the graphics were nearly always terrible on N64, as if they were spray painted or slapped together by watercolors.
So, in any event, you can play Soul Calibur on your XBox. Good for you. I'll still prefer the Dreamcast version. In fact, I've been itching to get another DC for some time, if only so I can play the emulators, and Chu Chu Rocket, and NFL2K1....and Soul Calibur, for sure. You can probably score a console and some games from local used stores or Ebay for $40.
Oh, and one last obvious point for me to make. Dreamcast Soul Calibur really was the best in the series. The Superunknown of its day.