I'm not a gamer myself, werner, but the reviews on reputable websites are unlikely to be trash.
"So then, I have to admit to like what I tested today. Overall gaming with PhysX adds a much more immersive experience to gaming. NVIDIA implementation as it is right now is downright good..." - guru3d.com
As for the drivers, while things have improved, it is an accepted fact that NVidia does have stabler drivers in general. Just so you know, I use a Sapphire Radeon HD 5770 myself, so it's not a personal preference. Just making sure, the OP makes an informed decision either way.
Actually, now that I think of it, if you're ready to save a few bucks, you can get a 5770. Should be plenty even on 1080p for "casual" gaming needs as you say. That way, your current 400 watt PSU won't be taxed either. As I've already said, 400 watts won't be enough for either a 460 or a 6850 on full load. And changing your PSU will add to the cost anyways.