I've had bad luck with USB cams, my logitech quickcam vision pro was very random whether it would work with iChat and FaceTime. sometimes yes, sometimes no. And it was $150!
They have a new one, the C910, but it's too expensive for me to dive in again.
So I bought an Apple iSight (the old external one) on eBay this week, should get here tomorrow. I'm hopeful a FireWire solution will work with all my apps, but we'll see. Hard to find good information in this area.
Kinda surprised Apple doesn't make an external camera anymore, since they sell a lot of Mac Minis, but I suppose they want you to buy their monitor, or an iMac.