Whatever chest freezer you decide on, the critical dimension is the area of the floor of the freezer compartment (the part not occupied by the compressor shelf). I've found that most smaller freezers have a interior front-to-back distance that is slightly too short to fit 2 kegs front to back with perfect alignment. That means that you end up having to stagger them slightly. Just an example of what you have to consider - bottom line is always measure the interior dimensions first, then work out how many kegs that will fit from there.
Also you can consider using the smaller (2.5 or 3 gallon) kegs on the compressor shelf - these are more expensive since they are less common, so you might not choose to spend the extra money to squeeze one more batch into your system. Right now I'm using the shelf on mine for the CO2 tank, a 6 pack or two, and some damp-rid. I have enough room for 3 full size kegs and someday when I'm done buying other brewing equipment I plan to pick up a pair of short kegs myself to get that 4th tap online
Finally, if you look around these forums or teh interweb, you'll see references to collars on kegerators. Very useful to build one a) because you might not have enough vertical clearance for the kegs as is, adding a collar lifts the lid off a few inches, b) you can easily drill your tap holes through the collar, avoiding the risk of hitting a coolant line in the wall of the freezer, not to mention being a good place to attach manifolds or regulator assemblies, and c) you can always remove the collar and turn it back into a freezer later on if need be.