If you can find a 3rd 360/PS3/ Wii game GS will slap on an extra $10 in credit. I also suggest using the funds to reserve a "Power Trade" game (even it you move it later), that 25% added on can be sexy.
As for Street Fighter: AC, you should sell it on Ebay/etc.
Many GS stores stopped taking original XBOX games, since it goes for @$15 in store you have no shot at receiving anything near the $20+ you can get for that puppy online.
yea wat they said,lol. Gamestop is starting to become a rip off for trades. Depends on teh game though. i only trade in games ther now if its important need like SF4, im gunna need to trade things in. Otherthen that. Sell it on here or Ebay