Quoting drasnor

The problem is that some of the latest batch of games are coded for only DX10 (e.g. Halo 2 PC). This trend can be expected to continue.

-drasnor

So you are saying that Halo 2 will not play at all on an XP machine or even a machine with Vista that only has a DX9 capable card such as a 7900GTX? If so, they don't want to sell many copies of their game then. Because there are many, many machines out there that fit the above criteria and probably more so than fit their Vista/DX10 card scenario.