Right now Nvidia has publically announced (after quite some delay) that they are experiencing some major problems with their current set of Vista drivers, resulting in poor performance and instability that may cause various games to freeze or crash. This does not affect all users as it's based on the computer's hardware, driver version, and games played, but they hope to work out all the bugs soon. I would expect a reasonable fix within the next month, so I would not avoid Vista just because of that current compatibility issue.

As to the gaming aspect, aside from the above I would not foresee any problems with playing Half Life 2 or similar games since the intended graphics card exceeds the expectations, as does any modern processor.

Hope this helps,
John