They don't render things "better" - they just do it faster, depending on the program you're using. Some programs will be faster with an AMD card, some will be faster with an Nvidia card.
However, amd does handle a multi-monitor setup much, much better. That being said, if you're going to stick with Nvidia, be sure to get a pair of 4GB cards - I recommend the 670. (The 680 is a bad pick, because it's only 5% faster than a 670, but it costs 25-30% more.)