If you have the juniper XT version and are a novice overclocker ( im asuming you are because you are using ati catalyst and not rivatuner ) the best way to overclock any brand of the 5770 cards is by using the MSI afterburner software ( linked at the bottom ) even if you do not have the MSI version of the card it still works great, but you will not be able to up your voltage. best part is that it is powered by rivatuners engine which is which is the next best thing to riva it self.
I personally have the XFX version and I was able to get mine to 960 core and 1445 memory ( thats the cards cap ) with the afterburner software. I have played 6+ hrs of aion at max settings ( FYI-aion uses cryengine developed for crysis ) without a single problem, i have since reclocked it to 900/1445 due to the fact that the 3-5fps im gaining from the +60 to core are not worth the cost of the heat the card gains from the overclock. With these clock settings i do not see any temps above 55c using manual fan control set to 70%-75%
Also it is a good idea to get gpu-z, its very good as a second check to see if your clock took to the card
In aion I see about 60-80fps in highly populated areas and about 110-130fps in other areas, love this card, and it will only get better with new drivers.
GL with your OC'ing.
http://event.msi.com/vga/afterburner/download.htm
system:
AMD PII x2 550 black @ 3.9 on air
Gigabyte ma790gp-ud3h
cm v8 cpu cooler
cm scout case w/ 3 120mm & 2 140mm fans
XFX HD5770 (juniper XT) 900/1445
4gigs patriot extreme memory
OCZ 550w psu
and some other junk lol