What's the safe OC for GTX 260 core 216?

My XFX black edition is OC'd to 666MHz (core) and 1150MHz (mem)... you can tweak it even further but it is an iterative process, and you need to test your own system stability to make sure it's "safe".

I have read of people overclocking them to ~700MHz, but you definitely need to keep an eye on fan speed/temp.

does it depend on what parts you have for your rig?

700Mhz on the core and 1230Mhz on the RAM fairly easily with my XFX GTX260 Black Edition.

I sometimes put it up to 720/1250 without problems.

Linked clocks(I could probably get more out of it if I overclock that seperatly but there is no real need), fan at 60%, no vmods. ~40C at idle to 65C under heavy load.

Although with my card as it is factory overclocked to 666Mhz may be higher binned and have a higher voltage in its bios. Also some cards are better overclockers then others so YMMV.

Normal overclocking rules apply, get some stress testing software(3D Mark, Furmark) and work your way up gradually(5-10Mhz at a time).

It varies, start off at stock and work your way up, a slow process but ultimatly worth it.

Go up 20Mhz incriments till you pass 670Mhz, then 10Mhz to 700Mhz then -5Mhz beyond that once it becomes unstable, lower it a notch.

Then start raising the ram in sensible incriments. Its mostly trail and error till you get the right balance.

All GTX260's should be able to do 650Mhz easily so you can start from there if you like.

Just for a reference my gtx 260 (216) vanilla is at:
Core: 745
Shader: 1510
Memory: 1025
Fan: 89%
Temp: idle 48
Temp: load 77

All gtx 260 if properly overclocked should at least hit 700mhz on the core. I have taken mine up to 777mhz for vantage and some crysis but decided to drop it back down after instability in Empire total war.

Good luck overclocking that monster!

+1, I suggest trying all those softwares, they are excellent for testing and overclocking. Personally when I used to overlock a little, I liked the ATI tool since it has a nice to use interface with a 3d object running as you overlock so you can see if there are any artifacts or glitches right away.