The baseline integrated graphics won't make a difference as the 21.5 features a Nvidia 640M. GTA V is also not released but should run on low-mid settings at least, most certainly if you get the new haswell iMac which won't be released until later this year, it'll most likely feature a 740M.
750M for the next step up from base will undoubtfully play the game on medium, but as I said, the games not released, so my opinion is just speculation.