It should be possible. I have a gigabyte soc running factory oc at 950. It get's quickly unstable though if i try to oc it further, but i can't up the core voltage so that might be the problem. I have heard from people they reached 1000.
It's hard to say if and how much it can be oc'd, every card, even from the same brand and model is different and will reach the point of instability at a different speed.
I like the cooling on the MSI better than the EVGA though. Only plus point EVGA has is the lifetime warrenty to 3 years from MSI.
The Zotac guarantees you 950 MHz and comes w/ BF3 ... solid deal tho I ahve no experience w/ Zotac
The Giga is a reference card w/ stock cooler and likely stock VRM
The Galaxy is a slight OC, most likely w/ stock cooler and VRM
So of the 3, I'd do the Zotac .... EVGA, MSI and Asus are the most common;y recmmended. I have seen posts w/ issues on the Twin Frozr where the heat pipes hits some MoBo heat sinks. EVGA is "personna non grata" in this household after 20 months or arguing for them to honor their liftetime warranty on a 4 month old card.
I own two of them and speaking from experience, I can tell you that they're the fastest 560 Ti available, they OC much higher than what they already are from the factory, they stay cool and quiet, and look awesome with the Twin Frozr III on board.
And in answer to overlocking the EVGA SC or the MSI cards:
Yes. I used to have a pair of EVGA GTX 260 Core216 SC cards and I overclocked them probably another 10% over factory and they remained stable. My 560Ti Hawks are sitting at 1025 Core / 2050 Shader / 2185 Memory and are stable. Several passes through 3DMark11 with no artifacting / freezing / crashing / etc.
I'd like to get the zotac but im concerned about the rebate (ive heard they suck with rebates), the sli, somehow i read somewhere it would fit on some boards or something (would iton mine), and the overheating... it runs mad hot... i dont want it to fry.. is zotac good with returs?