I was about to say the some thing as Slack, ATI have too many compatibilities issues.
If you wanna play a game you want to have fun right ? Let alone using a PC there's already many things that could go wrong (comparing to consoles) as your OS needs to be one, than your RAM, GPU memory, etc.
I've had an ATI, it was a good GPU and somewhat cheaper than NVidia, I think, but had some troubles too. Sometimes relying on patches made by players, not the developers themselves.
And for once, most games are built on a Nvidia structure (whatever that means), so it's only natural it has much less issues on an Nvidia card.
Just to exemplify, the Unreal Engine. It is optimized for Nvidia, and that doesn't mean that it won't work well on ATI, but if a problem is to show up, it's more likely an Nvidia card to be fixed sooner.