Well, if you're talking about fermentation temperature it's one thing, lagering temp is another. And if you want it in the same fridge as you're serving beer from, you need to be sure that you like serving your beer at the temps you need to use.
Fermenting lagers is generally in the 48-52 deg F range. Depending on the adjustment range of your kegerator thermostat, its highest temperature might not be that high, and then you're stuck with an external temperature controller.
The lagering phase itself can be done in a standard serving kegerator. Lagering is generally better at lower temps, so I'd try to set it as low as you prefer to serve your beer. If you don't mind serving your beer at 35 degrees and letting it warm in your glass, then I'd shoot for this.
But I generally recommend a fridge dedicated *purely* to fermenting. If you're not temp-controlling your ale fermentation, you should be. Spend less time worrying about over-sizing your kegerator (at higher cost), and get yourself a $50-100 fridge off craigslist and external temp controller to cover your fermentation.
