GTA V is suspected be as demanding as IV.
My GTX 660 runs skyrim with about 12 Graphics enhancements on at 80 fps. So a very strong GPU isn't really needed.
GTX 680 is very expensive compared to the performance. I'd go either Radeon 7970 or GTX 670. GTX 670 is only 5% behind GTX 680, so not worth the extra bucks.