If you plan to get a new motherboard, I would consider changing to an intel platform (H81 tends to be the cheapest) and a i3 or i5, then no overclocking would be necessary to get the most out of your GTX 760.
Yeah I agree with this actually. OP, just stick with your current mobo until you can afford a new CPU as well. The A8 goes up to 3.9ghz boost and overclocking would get you a 10-20% performance boost over that, not worth the investment. You should be able to get a small OC out of your current motherboard regardless.
Your A8-5600k may hold the gtx760 back a bit (this is entirely game and situation dependant) but it's not like things would be terrible for you. 99% of games out there you would get 30+fps which is fine and playable, and you would even get 60+fps on many games as well. Get the 760 and just enjoy, save your money until you can upgrade proper