You're going to restrict the performance you get from those GPUs by going with those CPUs. You'd be better off with a Pentium or Core i3 along the same price range. The new unlocked Pentium G3258 is a serious contender, at $60 at Microcenter or $75 at Newegg, with a motherboard at about $60.
The AMD APUs are more for situations where the user can't get a discrete GPU. They're seriously slow CPUs. I've used them frequently for family PCs. My mom has an A4-3400, and my HTPC is an A4-5300. They aren't real gaming machines because the CPU is just too restrictive.
The AMD A8-5600 paired with the GTX 770 would give you better performance if the game is not CPU dependent. If the game is CPU dependent you would get the same performance from the R9-280X and the GTX 770. The CPU is going to be your bottleneck in most cases.
You can afford these GPUs, but you're going with a piss poor excuse for a CPU? APUs aren't really meant to be paired with high end GPUs, lower, much lower end ones for crossfire work... but there are better alternatives. If you already have the A8, do yourself a favor and get a good CPU and either of the GPUs. Save up more money if you need to.
You're going to restrict the performance you get from those GPUs by going with those CPUs. You'd be better off with a Pentium or Core i3 along the same price range. The new unlocked Pentium G3258 is a serious contender, at $60 at Microcenter or $75 at Newegg, with a motherboard at about $60.
The AMD APUs are more for situations where the user can't get a discrete GPU. They're seriously slow CPUs. I've used them frequently for family PCs. My mom has an A4-3400, and my HTPC is an A4-5300. They aren't real gaming machines because the CPU is just too restrictive.