Okay first and foremost. Vista is only a Beta OS. It still inst Released to Manufacturing yet. So you will be messig with a work in progress till it gets released next year.
So the answer to your question is a doulbe edged sword. Vista is stable. It does run most apps woth little to no problems. But some gmaes and some apps do have issues. that is for the game and application developer to fix not M$. That is a common mistake. Everything thinks it is M$'s fault if a app doesnt work with Vista.
Vista is coded totally different than XP and therefor not every piece of software on XP will work on Vista. Not until they patch it to work on Vista.
As far as the Direct X question. DX9c is the latest version. I have not heard of no DX9L. That is a myth. I use the Most current DX9 (Oct SDK) and it still isnt labeled no DX9L or anything like that. I currently run several games on Vista with no problem.
Doom 3, Warcraft 3, UT2004, Halo, PainKiller, Starcraft, Doom 3 ROE, Warcraft 3 Frozen Throne. I have run into issues with Steam and Steam games. The Steam client doesnt run on Vista.
So if you play Steam games then you will have to wait till Steam can fix their problem between the applicaiton and the new OS. Some other games do have some minor issues. But nothing major.
Also know that DX10 will not be released for XP.
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