Hi,
Microsoft has no plans to get DirectX 6 working on NT 4.0. NT 4.0 machines are stuck with DirectX 3 - i.e. no AGP memory. DirectX 5, which allows AGP memory, is for Windows 95 OSR 2.1 and Windows 98 only. AGP and VPE were the big changes between DirectX 3 and DirectX5. Neither of these features work on NT 4.0.
On the bright side DirectX 6 will work, or least is supposed to work, with Windows 2000 ( previously NT 5.0 ) and Windows 9x, allowing the use of AGP memory on both these platforms.
If I've made any errors here, corrections would be appreciated.