SubZero, you realize the links you gave are to sites where people rate the video cards? A real comparison would look at the benchmarks.
However, the real answer:
Compared to Windows XP, Macs are pretty bloody bad for gaming. Yes, you can run chess and a few other basic timekillers natively, but any serious games you'll need wine (yay for mooching off Linux, they actually know what they're doing!) for, because Microsoft is willing to pay a fair bit of cash out to make sure games are made exclusively for Windows. Wine does not work anything close to perfectly with most games; however, it's good enough. If you want a gaming platform, Windows XP is still the OS to run, followed at a respectable distance by Linux (after all, the games developed for Linux, like OpenArena and many others, are quite a bit ahead of those developed for the Mac; besides, wine is a lot easier to get working in Linux in the experience of people I know) and then by Macintosh. I'd put Vista just barely ahead of Macintosh; quite a few Windows XP games won't work on it anything close to easily, and it has ridiculous system requirements that will put a strain on most midrange PCs while running newer games.