Dear Friend Gwion:
I would sell your xbox (and dont undersell...get the price that is worth)and use the money to get a accel card for your amiga. i would buy the accel card from amiga kit, this way you make sure you get a reliable card and don't have to worry about being ripped off by ebay scammers.
But if you want...you could always still buy from ebay, but be careful.These cards, on ebay, usually go from us$ 200 and up, all the way to us$500.
good luck!!!
rednova
-mobilis in mobile-