Headphones, webcam and speakers

I have just purchased the above for my Mac G5, which is a PC CPU. Questions from a total layman: (1) There is one port on the front for headphones, but my headphones have two 3.5mm jacks. Can I plug them into ports on the back, if so which, do I rewire them into one jack or is it back to the shop? At PC World, there's no info about anything, so you buy these things blind. (2) I think my Logitech LS21 speakers should work on a G5, assume it's the Audio in and out ports for a teleconference, or is it the Optical digital audio in and out ports? Is the former meant for radio, the latter for video, do I have to keep switching them around? (3) Which port takes the webcam, it's a USB connection?

Many thanks!

Roy