A simple motherboard question

Not a problem.
You can get a motherboard which supports ATI Crossfire but not nVidia SLI, and still use a single nVidia video card and no ATI card. Just remember that you will not be able to use two nVidia video cards in SLI with that motherboard, and that if you ever want to use two video cards, you'll need to buy two ATI cards or a different motherboard.

+1^
Using a Single Nvidia card on an ATI chipset or vice-versa is OK,but for using SLI you must have a SLI board and for using CrossFire you must have a CrossFire board,(there are some hacks which allows you to use SLI on a CrossFire board or vice-versa but they are not recommended   )

Some of the X58 boards allegedly support both.