As you up the core, the shader will also increase.
So you only need to adjust the Core and Memory.
And wow, your memory is already massively overclocked. 1100 MHz would translate into 4400 MHz effective memory clock (GDDR5).
So you should up your core slowly and stress test as you go up. From what I've seen furmark alone doesn't work too well anymore. You could be totally stable on Furmark and then crash on a DX11 app.
I used Furmark, Tessmark (DX11), Heaven 2.1 (DX11) and a few other benchmarks, as well as Crysis Warhead on Enthusiast settings at 1920x1080 and 8xQ AA
the card uses GDDR3