I would export at "full quality" or using "Expert settings" I'd choose uncompressed, compression "none" or Animation. The resulting file will be HUGE if you choose uncompressed "none" or animation.
Then open this uncompressed video (or full quality videowhich I think is DV) in another editor (MPEG Streamclip or DivX encoder) and convert to AVI. I'm not sure what codec you want to use. DivX? (I don't know what xbox uses.)
I'm trying it with the default settings if you do Share>>Quicktime>>Expert Settings>> (Select AvI from dropdown to left of options) >> (Click Options and then move slider to Best) Should be preset to 90 KB/s.
My Best advice, is take a small clip and share it with different settings, and see what changing them does to the clip, if anything noticeable, then adjust accordingly. Remember without some fancy filters and re-rendering, you will not get a clip to look better than the source file.
Okay so 90 KB/s is cruddy...go with something higher by at least three.
Brian