I don't see it playing GTA 4 very well. Also, your laptop slowed down because the CPU couldn't handle it. The GPU can slow down a game, not the computer itself. The GT 720M is a class 3 which means that while it could play minecraft, GTA 4 would probably only be playable at low settings. Perhaps telling us the make and model of laptop you're looking at would be of more help than just saying "i5 with a GT 720M" because there are other factors to consider as well.