Haha amigo I've been keeping my fingers crossed that my 2009 MBP with a 9600m nvidia chip will play the witcher. Unfortunately, that means your MBA has a 0% chance of playing the game.
If you read the other witcher thread in this forum, you'll see someone with a late 2008 Macbook aluminum (i.e. the 13") and a similar on-board graphics chip was hitting 9-10 fps when entering the game.
Sorry man... I hate saying "upgrade", but that's really your only option (probably my only option too).