The Linksys ProConnect KVM switches work very well, in my experience. I have the 4-port version with 3 computers connected. One computer runs at 1152x864x85hz, and the other is 1280x960x80(?)hz -- and I cannot see any image degredation.
The switch was about $85 on onvia.com. Onvia also has nice cable bundles (made by Compucable) for $8-$11. That's a bargain for a high-quality cable with mouse, keyboard, and video in the same bundle.
I'd avoid the cheap, mechanical switches. The ones I've seen in the past considerably lessened the quality of the video signal passed through them. They also don't properly simulate an 'always connected' mouse and keyboard, and thus they are somewhat of a risk... and could potentially damage your motherboard's PS/2 keyboard/mouse ports. Or they might not... but why risk damaging your nice, expensive motherboard just to save $50 on a KVM switch?