I swear DC is teeming with Apple heathens these days (sorry, can't help bugging you a bit :P)
There's a demo for Portal, but its Mac system requirements are significantly higher than those of Portal 2, so you may not even be able to install it, thus not really concluding if your computer can run Portal 2. But I understand the Source engine was greatly optimized in Portal 2 so the game could run well even on integrated graphics chips, provided they're new enough. You should complete the first Portal before playing the second one, anyway :D