I'd refurbish it for a reasonable price and keep it as a back up laptop. Then again, having you having an Air, this MacBook probably feels like holding a bunch of bricks. That's how my 15' MBP felt when I upgraded from a 13' MBP.

I'd like to find a MacBook for cheap on craigslist and fix it up and keep it for helping connect some older Macs that don't support an AirPorti card to the Internet using Internet Sharing. One day, one day. :-/