Hi
That should be easy enough, just setup the 3G part in the router and you can set it to use 3G all the time or just when the ADSL line fails.
As a matter of interest, instead of having 2 NIC's on the server why don't you just use 1 and utilise the excellent firewall in the Draytek router?
It makes for a simpler network topology and takes some load off the server.