I have to admit, you can play some of the older Valve titles really well. I played Half Life 2 Episodes 1 and 2 flawlessly on an Intel GMA X3100 (256MB?) on a Macbook in bootcamp'd Windows-- mind forget for most people.
But really, its not ideal. If you want to play oldie titles well, why not pay for a high end card-- even if you get one from the lower end of that food chain.