Will look up GChat - am guessing it's very similar to Skype?
I'm probably only going to need to phone over the next couple of months but they're quite lengthy calls. Home phone is with Virgin (until I escape their clutches in a couple of months) and iPhone's with Vodafone but I didn't want to pay for a whole year's international calling when I wouldn't need it, iyswim.
Update: I used Skype and it was clear - lost connection once to SF; to Hong Kong it's pretty dire, the other phone call I also ended up making: voice distortion and dropping out repeatedly. So I used Viber and it worked a treat!
My internet connection is 'meant' to be pretty fast but I haven't run a test recently to see the true speed.
I will definitely use the phone card suggestions though if Skype doesn't prove reliable.