Personally, B, I would get it for computer and then buy one of
these - it's what I have - - (ah hell, they even have Xbox adapters now)
There is a messy compatability issue with Mame versions and ROMS - you may need older versions of MAME for certain Roms - I suggest getting two or three versions, some older versions - and if you can download them off Kazza or some filesharing FTP site, a shitload of ROMS and just see what you can fire up -
Even when I had 1200 ROMS, I had some that didn't work on the latest version of MAME.
Regrading the hacked X-box - that's a cool idea, and I will try that when the 360 comes out and I got this POS lying around - LOL - but until then - I'm just gonna play Xbox and build a dedicated PC for MAME - I also have Stella and every Atari 2600 game on it, so THAT is cool -
RB