The R9-270x is basically the same as the 7870 with an o/c, the R9-280x is the same as the 7970 GHz. There is no current equivalent to the 7950, but it would basically be the R9-280.
I would recommend the 7950 over the R9-270x as it's the next step up and are only around $200-230 USD and has 3GB Vram vs 2GB. Although >2GB is not yet needed for 1080p, it may become useful in future games or multi-monitors/>1080p. Just get one with a good cooler (like mine LOL!). 7950 overclock well. eg stock 800MHz, o/c to 1200MHz. But you'll need one that is not voltage locked for that.