Entirely depends on your spec and rig and whether you're online or not. GTA is not a graphic intensive game so much as it's a Cpu intensive one at the low end. I played a LOT of GTA on a 560Ti 1GB@768p (quick rig for ms while I had 970@4k), and it runs at about 40fps on low quite consistently, however turn the res up, esp when driving fast you start to get textures droppin so fast you're driving on invisible ground and can only see sea and skybox.
That's what happens when you run out of VRAM. Your 660 has twice as much, so basically it will run well with 1 660@1080p, but not at high settings without you disabling the VRAM limiter and getting objects disappearing when moving at high speed. SLi is going to improve your shader function but won't really help the fact that your total VRAM amount is what's holding you back from high settings.