Look into the Gigabyte 290 OC Edition, it a top tier GPU and even though it is oc'ed already you can get more out of it. Don't even worry about the memory clock as it's more like making the card look more pretty. Since 5ghz with a 512bit bus is already crazy. Just focus on the Core clock.
I was reading a few reviews and the cards that come with a memory OC is more so to trick you into thinking you're getting more when you're really not going to see any difference.
But as it is any OC Edition is going to be better since they are binned and have the better batch of components. The Gigabyte 290 OC Edition is what I ended up going with after 2 months of redding reviews, asking around, ect. The reason is it has the best PCB and the best cooling system. Though it can be debatable, there is no doubt Gigabyte uses the best cooling method and components. The Wind Force 3 cooling system really is amazing. It also covers the vram and other components that are not cooled as well with other brands. The 2x copper PCB is a good enough reason alone.