Well, the 330M that I have (512 mb) is keeping up with Oblivion and RSII very well. I'm satisfied with the 330s performance in both games, althought the 330, like the 320, is considered a "mid-range" graphics card.
If you want more detailed info on the 320,
here's a link to that includes some game benchmarks.
That said, one expensive lesson I learned a couple of years ago: no matter how good your graphics card is, no matter how much ram you have, without a good processor, you just bottleneck. Which is why I spent the extra money for the i7.
In all honesty, I love(d) both my MacBooks. They weren't perfect (although my current one is
near perfect) but they were far superior to any PC that I've ever owned or seen. Others, of course, have a different opinion. I'd try to get a loaner, if you can, for a few days, just to test a Mac out.
I'm looking for a good 2009-2010 RPG to test out my new i7, just to get a feel for how well it handles other games ...
~ Dani ~