Assuming you have the small form factor with only 1 fan like I have. Honestly, in my experience, going another 100mhz is easy and quite sustainable in my case. But do not tinker too much with memory. Even slight 50mhz increase would shake things up.
Forget about overvolting. Since the nature of the design forsake additional power, overvolting is out of the question. You're welcome to try though. Bear in mind, at full load, the whole thing sucks about 70watt. You have another 5watt as per PCI power limit, give or take, to play with.
After awhile, I choose to revert things back to default factory clock. Zotac already OC it a little bit out of the box. The difference in performance is negligible. More often than not, only a mere 2-5FPS...