GTA V is quite a while off yet, nobody knows what the requirements for it will be. GTA IV is a very badly optimized port to the PC. Even on systems better than what was top of the line when the game came out, it still runs poorly on high settings. Maxing out GTA IV also requires more than 1GB of video RAM, something most 6870s don't have. If GTA V is anything like GTA IV you're definitely not going to max it out with a 6870, but you probably could still get better than console performance.
There isn't much point in buying a card specifically for a game coming out in the far future, we simply can't tell what a future game will need. If you want to be more 'futureproof' get a 6950, or wait for the HD 7000 series, and pick up something like the 7870. If your main purpose is GTA V, you probably will want to wait until the game's requirements are out closer to the release date.