I can't remember if you were runniung a GTX or not.. But anywho, my card on stock cooling can get to a stable 632/1050. Temps aren't that bad either. But just like a CPU overclock, the RAM has to be in tune with the core clock. I would suggest leaving the GPU where it's at if you're happy with 600, and then lower your RAM. Otherwise, you could raise the GPU and leave the RAM.
I suggest aftermarket cooling though. The dual heatpipe on the stock cards are just weak as all ****. I'm buying the Thermalright HR-03 Plus today.. They get really good reviews and drop temps a good 30°C