One other thing to note on 6.1/2/3 vs 6.5
DirectX 8 vs DirectX9
One reason to stay with 6.1/2/3 is older computer support for cards that don't support or run well on DirectX9. Yes, there are still lots of those out there

Especially if you deal with businesses (government especially) who don't upgrade computers as often as gamers do.
On the other hand. DirectX8 isn't officially supported on Windows Vista, especially programs that use the Dx8vb.dll (or something similar).
Also if your targted use base has a large number of Intel (Piece of Crap) onboard cards then DirectX8 will be better. Even though their(Intel's) marketing says their newer cards fully support shaders, they are still very crappy at it.
As a profesional developer who has been using TrueVision3D since beta (6.0?) I have a lot of experience dealing with a wide variety of computers using TrueVision3D.
From a developer's perspective 6.5 kicks butt on earlier versions

Cheers,
paul