I've got an 8400, and a 4600. Both with upgraded video cards with DVI. 'WIRED' USB driven Mice & Keyboards seem to work fine, (stand alone), no KVM switch with use of a standard PS2/USB adapter plugged into the PS2 ports of the towers. Introducing the IOGear GCS932UB 2-port DVI KVM Switch = a different story. No recognition of mouse or keyboard.
I found this post and wasn't happy to see that some folks actually had to go out and get a USB powered hub as well ... what's the point if the switch won't work?!
There are 3 types of USB-based kvm switches.
1) Hub-base, it will work for almost all kind of USB devices you connected to the KVM switch, but there is no "hot-key" selection and no mouse or keyboard emulation at all, So, the switching time is long, because every time you do th switch by push switch button, those connected keyboard or mouse work like you physically unplug and replug in between computer systems.
2). Emulated type KVM switches, (just like your IOgear KVM), This kind of switches do have hot-key selection and switching fast (when it work), but it normally only can emulate your keyboard and mouse as "standard keyboard and mouse", so most of time, if you used any converter between your keyboard mouse and KVM switch, it will not work correctly.
3) Using Dynamic-Device Mapping technology, which will work with most of USB keyboard and mouse without emulating them into just as "standard" devices, but keep the original features/functions of the connected USB devices to feed them to all the connected systems all the times. I strongly suggest you can try this kind of KVM switch, if keyboard mouse switching time is very important for you and special features of your USB devices are important to you.