Start here, read this. http://www.tomshardware.com/2007/12/03/best_graphics_card/
The 3850 is a better VALUE. If your budget is tight, I suggest this card.
The 8800gt is slightly FASTER.
Many will say ATI has better picture quality, I would tend to agree with them that ATI cards do generally have a better picture quality than nVidia cards.
Against these 2 choices, it's mostly personal preference.