Is Nvidia better? Not really. Amd used to be known for bad drivers before, and hence Nvidia became popular among enthusiasts ever since. Amd cards are on par with Nvidia ones, except that they lack physx which can be achieved on your cpu via a small tweak.

The pc version of gta 5 won't see the light of the day for at least another 6 months or so. It also depends on what you consider "good enough". If you're going play on 1080p or below, you should be fine with most settings cranked up( no AA). Anything above and you'll need cf/sli.