The one piece of advice I'd give is to avoid the cheapo Belkin USB ones with integrated cables. They are absolutely dire - we tried two, and they'd randomly switch back to one machine while in use, didn't work at boot, basically a complete pig.
I've got the PS2 variety of it, and its been fine, so if you are still using PS2 then that may be an option
David - DVI KVMs are beginning to appear, but I guess you'd pay the (large) price tag for them, and I don't know whether they would be Dual Link...