I have recently purchased two kvm switches for my office, after quite a bit of shopping around. I actually called my sales rep at one of the online vendors I use to ask his advice because I had been stumped.
Here's my advice:
I. Always consider going with a model with more ports, there's always may come a point where one or two more ports may come in handy, but not necessitating a new kvm switch.
II. Belkin and IOgear are both great brands for kvw switches, just make sure to check out user reviews on cnet for particular models. Sometimes they will try out a new feature that won't work so well.
III. Make sure you buy the right cables, if they don't come with the switch.
IV. USB support is almost essential now. One of the models I have is USB, and the other is PS/2, but the USB one is definitely preferred all of our newer computers that don't even have PS/2 ports. Port replication is good for PS/2 switches.
V. Video quality can also be a concern. This is the reason I went with namebrand models.
Most KVM switches work to some satisfaction, but the better ones are much easier to maintain and use on a daily basis.
Good luck!