If you get one from XFX or Evga you can OC as much as you like without voiding your warranty. You may also replace your HSF and not void the warranty.
If you want a guaranteed OC grab one from Evga or XFX that is factory overclocked. XFX has an OCed card that is @ 640 out the box for 405usd @ Newegg now-
That card was put up against a GTX 280 @ 700 on the core here-
A GTX 260 @ around 640 on the core should be enough card for most folks. The one [H]ard tested did 675 with zero issues. Firingsquad got one that went all the way to 712.
It is luck of the draw and I doubt there are a lot of cards out there that won't do 640. nVidia often releases cards at lower clocks first them ramps them up later.
You can always go SLi later too. GTX 260s might be below 350 or less by then.