Look into a app called Tango i forget i it is for Droid, but it works for Iphone 4.
Its can do video over 3g and wifi.
Facetime can't do over 3g Yet, unless you jailbreak phone and install a certain app to do so.
That being said facetime is only for 4 generation products from Apple.
Skype should work for you as well, what issue you having?
Just remember for Skype and or Tango they also need to install that app on their phone in order
for it to work, both users need to see each other.