None can. The best ones have their displays maxed at like 90hz, but the actual circuit board can receive a 120Hz signal. There's a lot of issues around it as well.
There's a lot of confusion and misinformation out there, so what I'd suggest you do is check out 120hz.net to learn all you can.
I've spent the better part of the night trying to understand whats going on with the monitor overclocking thing. I don't think I understand it at all.
There are quite a few people claiming to have overclocked to 120hz, but apparently you need a special PCB (the "2B" revision) of these panels.
I dunno. I am pretty happy with 60hz IPS panels to be honest. The 3 Dell 22" I have look great to my eyes. Awesome color at any angle. I played a bunch of Unreal Tournament 3 on one of them and it feels super fluid to me. I doubt I am in need of overclocking refresh rates anyways.
The only issue I am going to end up running into is I still think I want to do a multi monitor setup, but I don't think you can do mismatched monitors for nvidia surround. Then again, the appeal and charm of that eyefinity/nvidia surround will likely (hopefully) be dwarfed by Oculus Rift kit coming in a month or so.
The biggest kick in the pants for nvidia surround so far has been just the sheer power needed to run @ 5670x1080 (6.1mil pixels) and the lack of support on some of my favorite games. Getting that 27" in the middle running @ 2560x1440p (3.6mil pixels) is going to take a huge load off the system, have a higher pixel density, and the best part is, 1440p is pretty much universally supported.
Decisions, decisions.
I'm just glad I have a little bit of room in the budget for finalizing my monitor setup. For awhile there I really was thinking of going for a Titan. I think at that point I'd be forced to game on a 19" Packard Bell CRT.