I be honest when doing GPU OC's I like grabbing the highest "stock" OC from an manufacture that sell the same model GPU and than set it at that as a baseline to test. And than work up from that.
But if your asking if you can just OC the GPU clock speed and not the memory, from what I recall when I OC my GPU years ago yes you can. Just stress test it afterwards. I would also do a quick google search on what people got when they OC their 4890's so you have a rough idea of what it's capable of.
If your asking us if this setting will be stable, we can't tell you as every part is diferent and is able to handle different things.