Is New Vegas enhanced for amd or nvidia video cards or does it matter?

It does not matter. If you have any decent video card from the past 3-4 years, you should be fine. Key there, is 'decent'. Don't expect high resolutions with all the bells and whistles turned on, if you have a low-end cheap card or :ugh: an onboard graphics chip.

It is enhanced for Xbox...

It can matter what your PC will cost to run over time, and how much heat the GPU generates at Idle or under Stress...

Well, that gets into the whole "how does a computer work, and please teach me about all the little grainy details of case heat control and planning, watt usage and hourly costs of different individual cards and chips by different vendors, power supply capacities, etc, etc..." which could take 100 posts in a whole nuther thread, to do any justice to if he's a newb to such things

Assuming he is capable of choosing a card from either vendor that will actually run in his PC and won't melt it down into a pile of molten slag, and that he isn't horribly concerned with whether it costs $3.00 a month or $1.25 on the power bill to run... then the answer is still no, it doesn't really matter. Both vendors have a lot of cards that will run it just fine, the game isn't optimized for any particular video cards. Hell, it's barely optimized for computers.

But, if you are unfamiliar with all that technical stuff and want a complete training session, feel free to ask anyways.