270X is mostly just a 7870 GHz but clocked a bit higher. If you were going to go for a 270X I wouldn't spend more than $210-220 for it, otherwise I would consider a GTX 760 2 GB.
280X is a 7970 GHz. If you're willing to spring with a 280X I would go with one of those or grab a GTX 770 2 GB in the same price range.
> Also, I hear you can only change so many components in your PC before windows starts to act up and void your licence.
That pretty much only happens if you change motherboards, and it only forces you to re-activate which often works without a hitch.