MrYokeJust look for benchmarks of both graphics cards. You should also decide on a manufacturer.I'm mainly just looking for advice, because when it comes to stuff like benchmarking, and telling exactly what that means, it is completely out of my league.
Reading benchmarks is pretty straightforward. Try to look for specific games if you can. For example, http://www.gamersnexus.net/gg/1352-titanfall-pc-gpu-benchmark-full-version
Graphs pretty much tell exactly what you need to know. Most of the time it's in average fps. Higher the number, the better the card. If you can find specific benchmarks like these, they'll help you more than if you found maybe the 3dmark 11 benchmarks, but those are pretty straightforward too.
In general, higher number = better performance. Unless it's temperature. But you don't really need to worry too much about that.
Also at the end of the day, it's your decision whether or not you want the extra performance the 760 gives over the 660. If you do your research and you find that you're ok with having the 660 performance, then save money and buy that card or use that extra money to get a slightly better other part.