Current X2 Processors seem to be pretty variable in the amount of Headroom they're coming with. From what I've seen posted, The "Better" Toledo cores 4400 and 4800's have been reaching 2.4-2.7Ghz without too much trouble - (cooling dependent).
The Manchesters (3800, 4200, 4600) seem to have a little more room in them (making them much more attractive to OC'ers) but remember that they start lower and if you're unluck and get an underperforming one you will be stuck at slower speeds!
My 3800+ is living life running at 2.7Ghz, but I can bench at 2.8Ghz - Max OC (Widows Bootable but not stable at all) is 2850Mhz. But I'm watercooled and that can make a BIG difference.
I've seen plenty of posts with people running X2's at 2.6Ghz on air with no problems at all.