660ftw is my first nvidia, but i say dont look at 200 mark look at performance mark. you get 50% more fps in games with 660factory overclocked for a 30% of price. afterwards it doesno longer make sense to go ti as there you would get only 10% for another 30% of price. so 660 is the perfect performance price you can get.
if budget is issue go with 2gb i barely managed to max out 2gb of vram in skyrim with high textures, so you should be fine
i had buyers remorse when i bought 5750 when i could get for 20eur more 5770 with 10% more performance but i could live with it. but 50% more performance is completly different. you can max out all games and get average 40-45fps on ultra details, meaning you get 35-60fps in games. in skyrim i get 60-80fps and its totaly modded with hight texture and enb
also nvidia consumes much less power than ati so if you have older psu that might have degraded a little nvidia is sure way. again i had ati for 16 years so i am not bashing ati, just saying