The baseline Mac (a $599 Mini or $999 Macbook) come with a Geforce 9400m. While not an amazing 3D chip, it's much better then the typical Intel Integrated graphics chips on entry level machines from most PC vendors. Portal or Team Fortress 2 would play just fine on them, and it's been a standard graphics chip in Macs since late 2008.

MacBook Pros at the mid 15 inch level and up also have a 9600GT dedicated chip, along with the 9400m. They are due for a bump here soon, and will likely have comparable graphics options to similar PCs. Apple has on their mid and high range kept up with PC graphics options for the most part for well over 7 years now. Back when the ATI 9800 was a great card, Macs had compatible versions for the G4 systems.

*edit* One big thing Steam does is provide a digital distribution channel to the Mac, something that will likely be used for many casual games as well. Peggle and other PopCap games are already available on Steam for Windows, so I assume they will be out on the OS X Steam too. The casual market is one that will continue to grow, and now Valve will be earning a portion of that spending on the Mac side.