When I had one 660 non-ti, it could play almost any new game on the highest settings with 40-50 FPS. A few games such as Far Cry 3, Battlefield 3 and Max Payne 3 I could play in high detail settings in 1080p but these games also have an "Ultra" setting and the game didn't have good FPS in ultra. A 660 ti might not be able to hit ultra in these games either. When I got a 2nd 660 for very cheap (newegg $168; both are the Asus OC DCII Top) and put it in SLI mode, every game is playing in the highest possible detail settings with at least 55-60 FPS. I got both cards for just under $375 after a few rebates. A GTX 670 costs around $400. My benchmarks indicate that the 660 SLI setup is around 20% better than a GTX 680. If you are short on cash and the Asus 660 is on sale again, I'd say go for it and then in the future you could always get a 2nd.
A 660ti also has 192-bit DDR5 VRAM, but it uses the GK 104 found in the 670 and 680 as opposed to the fully enabled GK 106 in the 660. The 660 ti uses 2 PCIe power cables, the 660 only one. This means less power consumption and less heat. Both cards have 24 ROPs, while the 660 has 80 texture units and the 660 ti has 112. These last two numbers are a better indicator of a GPUs "horsepower" than their core count. This is a review comparing the two cards with two similar AMD cards, the 7850 and 7870:
http://www.eurogamer.net/articles/df-hardware-geforce-gtx-660-review
This site is usually biased toward AMD, but the FPS charts show that between the 660 and 660 ti on most newer games are fairly even in terms of FPS. The 660 ti beats it when the FPS starts to go over 120 in less graphic intensive games like Portal 2. In this comparison, the two cards are not benchmarked, instead they are compared based on what their hardware is theoretically capable of:
http://www.hwcompare.com/13296/geforce-gtx-660-vs-geforce-gtx-660-ti/
And a review comparing the two and other Nvidia cards with a table showing specs of each card:
http://www.hardocp.com/article/2012/09/13/asus_geforce_gtx_660_directcu_ii_video_card_review#.UYPF87Uqa4c
For $100 more, I don't think the 660 ti is worth it. On paper, it beats the 660 obviously, but in real world performance, I doubt you will see much of a difference between the two especially if you are using a single monitor in 1080p or 1920x1200. Besides, if you are planning on using multiple monitors and higher resolutions and need to keep the card under $300, a 7870 or 7950 would be a better option. One display, 1920x1200 or below and a 660 is the best bang for your buck.
Asus DCII OC GTX 660 for $189 plus a free copy of Metro Last Night:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121660