According to Game Informer:
For those who recently upgraded their PC video cards to support DX10, you'll be happy to know that both Episode Two and Team Fortress 2 will support the advanced technology of these cards, even if the PC is running Windows XP. On the low-end front, Valve still plans on supporting PCs that can only run DX8 applications. Valve's attempting to also support DX7, but couldn't confirm if it'd go back that far with technology.
Even though Lombardi wouldn't confirm a release date, the current schedule is to release all three versions of the packages in the Fall. Lombardi admitted it was a big undertaking to go multiplatform with this package, but the goal is to ship all three versions concurrently.