If I have Vista but dont have a DX10 card will I still be able to play Halo 2 or do I need a DX10 Videocard?
Technically, this is all you should need to run Halo 2:
Windows XP
pentium 3,4 or amd athlon 1500+
256 mb of RAM
64MB video card directx 9.0 compatible
(It did, after all, originally run on an xbox)
Yet for some reason, Microsoft recommends this:
Windows Vista
2 GHz or faster processor AMD (or equivalent)
1 GB of RAM
128MB video card directx 9.0 Compatible
So, to answer your question, no, you don't need DX10 but you still need vista.