I'm looking to link up my PC to my friend's Mac in America so we can have free - or at the least very cheap - phone calls, using either a USB phone or standard headset.

Are there and free or cheap packages that would enable me to do this?

I haven't used it but a lot of people rate Skype highly.

http://www.skype.com/newtoskype/

http://www.skype.com/allfeatures/skypetoskypecall/