650ti is not very close to 660.
650ti with Boost then very close to 660, but it is already max of what they can give for the 650 chip. The OC performance for 650ti boost is not that great considering the chip reaches to its peak.
So in other words 650ti non boost is still quite far from 660. Let alone 660ti.
So if you are doing not often gaming once in a blue moon type and play games such as those mmorpg that is non gpu intensive can go for the 650ti.
If you do game older titles and do not wish to break the bank, 660 will be a good card.
If you wanna game AA titles and do not wanna play on ultra settings and just wanna play somewhere at high settings, can go for 660ti. 660ti can consider a fast card and is somewhere close to 680 performance on a slower side.