oh okay, in most western nations 770 and 280x are same price and 770 is a little better.
there's no performance issue but it doesn't make much sense to use a high end card with a weaker processor as your system doesn't have good "balance"
To use an analogy
The 280x with a fx-6300 is like putting a Ferrari engine in a BMW, you won't notice but it's a bit odd
but you can use them together fine.