The standard Telstra Next G Wireless gateways aren't bad. They're a rebadged Netcomm router so not too bad quality. (
http://www.netcomm.com.au/products/t...igpond/3G10WVT)
I have one of these with an external 5DB desktop antenna down at the Common User Facility in Henderson. Works quite well down there, have about 7-8 people who have been working F/T via VPN back to head office for the last 12 Months. It's not the fastest thing in the world, but i works well enough.
As with Jesters suggestion these don't need the Sierra Modem, just slot your next G SIM card in and away you go. Don't really see how it would give you better signal strength but?