There is zero reason to stick with the drivers from Microsoft. They are provided simply as a means to get youup and running and to have Areo work.
If you're gaming, then you must run the drivers from ATI (or nVidia) for that matter. As there are optimizations and other tweaks in the full drivers over the WDDm drivers. It makes no sense to stick with the default drivers
An example, is my 3Dmark06 score was 4800 with the MS drivers and 5989 with the ATI 7.1 drivers. I'm running the 7.1 with an X1900XTX and though performance is slower (whihc is expected) only the game FEAR so far seems to have some issues. Every so often, when exiting the game I'll get an all brown screen and Vista is hung. I've gotten a few tray icons telling me my ATI card as recover from an error. This is only under FEAR.
I've played with Doom3, HL2 and play alot of Battlefield2142 with no problems. I'm sure the next couple of driver releases will only make things better.
Yea I don't think OpenGL is intergrated into Windows Vista like it was in Windows XP. I think support for it has to be provided by the drivers.
Well that true to a degree, but load base Windows XP and it will load drivers for you, but they are not the full feature set of the card. Check out this old article
http://support.microsoft.com/default.aspx/kb/283751
excerpt
The Windows 98 drivers are upgraded automatically to Windows XP video drivers, and the Windows XP video drivers do not have OpenGL support.
To resolve this issue, contact your video hardware manufacturer for drivers that are compatible with Windows XP and that provide OpenGL support.