Just started playing Portal (and now Portal 2) on my Mac Mini Server (the latest one with a Thunderbolt port). Plays well in my opinion; no lag or refresh issues that I can see.
The Core i7 will go a long way to make up for the graphical deficiencies. Start talking about more graphically-intensive games and you might have some problems. You also may have to drop the resolution to 1280x720 (720p) for the frame rates to be reasonable.
Playing under Windows will also help a lot. My MacBook can't play Portal under OS X, but it plays perfectly fine at 720p under Windows. And Windows, being 64-bit, can only access half the RAM in it that OS X can.
i've been playing it on 15" Macbook air via steam with no problems... well, apart from getting stuck on certain levels but can't blame the computer really