A 7950 will max out most games currently, while the 7870 may not. Get a 7950. Make sure to get one with custom coolers as overclocking will allow you to keep at max settings for a longer period of time. Btw, there is little to no way to future proof your system. The only advantage of getting a 7870 would be to take advantage of the savings now to buy a better, high end card in 3 years. By that time, your system may bottleneck a high end card anyways. Basically, spend what you can now, and upgrade when you need to upgrade.
I highly recommend getting a 670. It's around the same price as stated, and games will work better with it since they'll be more optimized for nVidia cards. Also, I also do believe they work better with Intel CPUs. People don't often go get a nVidia GPU with an AMD CPU or a AMD/ATi card with a Intel CPU. In the long run you'll be happier with the 670. Here are some of the best 670 cards: