You guys are looking at this the wrong way.
If I was buying a new GPU, I would for SURE get the GTX 750Ti over the GTX660. Why? Well the main reason is that the GTX660 costs around 189.99 usually, while I can find the GTX750Ti for around 149.99 or 159.99. That's a 30-40 dollar increase in price for the GTX660. And the OP might not want to spend that kind of money.
I also don't think the little extra performance you get from a GTX660 is worth the extra 40 bucks.
And comparing core count with these cards is like comparing the GTX580's cores with the GTX680's cores. The 580 has 3x's less cores, they are just more powerful but the 680 wins because it still has 3x's more while only being 2x's less powerful.
Same goes for the 750Ti. 750Ti's cores are about 1.5x's as fast as GTX660 cores. So basically, they have the exact same amount of cores. But where the GTX660 wins is in memory bandwidth. The GTX660 has 192bit memory bus, while the 750Ti only has 128bit. But the 750Ti also has a newer better way of dealing with memory, so that ameliorates the 660's 192bit memory bus a LITTLE, but not fully.
So basically the GTX660's memory subsystem is what makes it a little faster. Is that worth the extra 40 bucks? I don't think so at all.
Go with the 750Ti my friend. And you won't even need a 6 pin power connector and you won't even be able to hear your graphics card spin up at all. It'll be silent. The 750Ti should also get a decent boost by overclocking it. Only to a certain extent as it only has 75w of PCIe power to run off of and stock it uses 60w.