An alternative to buying a KVM is to use this software (free) called synergy:
http://synergy2.sourceforge.net/
Basically, you install it on your PC and on you Mac, and then you need to make a simple network between the two (you Mac will be network ready, and you can buy a cheap network pci card for the PC and an ethernet cable).
The software lets you move you mouse between the two screens and whenever the mouse is on a certain screen the keyboard will send the keystrokes to that computer, you can even copy and paste between the computers. A lot more elegant than a KVM switch (the only time you really use a KVM is if you only have one monitor and two computers).