A particular title people have been waiting a long time for on the PC has been Halo 2. Though readily available on the Xbox, the PC release was in debate for a long while and then in development for longer, where it currently is. However, in a fan-driven interview, many details have been released, including what sort of system will require it. The only real worthy point of note is that this game will require Windows Vista to run. Requiring an OS that may not be out for another year? I guess that rules out Mac and Linux, but that's no big surprise considering who Bungie works with.
Of other things to note, Xbox users will be able to play against PC users once it is released, and a Halo 3 is already in the works. The interview has a bunch of other questions, though most are light or absent of content regarding the game itself, but the points worth noting are summarized. Requiring Vista may change, but most likely not, as it's probably a technical reason they decided to axe support for earlier versions of Windows.