PC and a Mac on same monitor, is it possible?

Use a KVM. There are bajillions for sale and findable via Google/Froogle. You'll want to shoot for a DVI KVM (to support your monitor) and start using USB on your PC if you want to only use 1 keyboard and mouse.

Might need a new video card for the PC, too - does the current one support DVI?

Edit: I agree that USB keyboard and mouse is a good idea, but I've used a PS/2 to USB adapter with my older KVM so I could switch between my Linux box and my Powerbook. At work I've got both hooked to a 21" CRT and one of those MS split ergonomic keyboards.

Yes, good call on the new cideo card that supports DVI.

The only problem is if you get a kvm switch it will cripple your max resolution output for your monitor. And the ones that don't are so expensive that your almost better off buying a new monitor. I havn't shopped around for them in a while so maybe someone might know one that I don't that has the resolution of a 23" screen and fairly affordable. I know I would be interested in hearing about it.