Hi Shuttergirl!
"Virgin Seamless" which is a bus service takes you between the domestic and international terminals.
If you have a formal connecting journey where they check your bags right through to your final destination at your originating domestic airport (that is, one ticket - not a Virgin Blue booking and then a separate international V Australia booking) then this is free - if you have two separate bookings it's about $15.
If you have a formal connecting journey, when you check in for your first domestic flight to Sydney they'll give you a little info pack that will contain a map and a voucher for a Virgin Seamless bus to use at Sydney airport.
When you get to Sydney domestic, go to the Virgin Seamless lounge near Gate 37 where you can check in for your flight to the US and get your V Australia boarding pass, and then go downstairs and the bus will take you over to the international terminal, where you can head straight through into immigration.
Happy travels!!