Ati gfx on nforce mobo...?

Can I install an ATI grapics card on to a nforce 650i motherboard?

Ati gfx on nforce mobo...?

Of course. Any x16 PCIe graphics slot should follow PCIe standards and should adopt any PCIe graphics card, regardless of brand.

Crossfire and SLI are different. They are proprietary to ATI and Nvidia respectively.
Yep, I've never done it but I'm almost positive, then again I could be wrong, but I'm ABSOLUTELY POSITIVE (hopefully). However you can't run two of the cards surrounded by crossfire because Nvidia boards support SLI while only ATI boards support CrossFire, so I would in truth get an ATI board over an NFORCE board if I be you. Or just buy a Nvidia Graphics Card