A KVM (keyboard video monitor) switch won't affect frame rates at all. Some do have a maximum resolution that they can support though...
Video quality will be reduced a little bit, but that's actually because of the extended cable length and additional connector loss. If you use good triple shielded montor cables and keep the lengths short you probably won't notice the difference.
There are manual and electronic KVMs. The manuals are just a rotary switch and are usually much cheaper than electronic KVMs. They generally work ok, but do have a big fault in that they disconnect the keyboard and mouse from the system when that computer is not connected. This will cause problems if you boot that computer if it is not selected (it won't detect the KB or mouse). The electronic ones generally do not have this problem.
Belkin does make electronic KVM switches. Expect to pay about $65 for a two port switch and $85 for a 4 port. The cables you will need will easily match that as well. For a 2 port setup you will probably spend around $100.