In the place where i work i have a 2820 and a BT ADSL connection

The problem is that i am based on a line virtually at the maximum of the ADSL limit meaning i get a very unstable 1Mbps.
As i am unable to get a better wired connection i want to add a 3g dongle (with external arial) to the router.

As the download limit on 3g is quite low is it possible to set a few of the most important computers to use the 3g connection and all the others use the slower more unstable connection?

Cheers
Lloyd