On 2.0 you can edit the gateways and weight them appropriately based on how fast they are. For instance, my cable is about three times faster than my DSL, so I have my Cable set to a weight of 3, and DSL to 1.
If you are on 1.2.3 there is a trick you can do the same way, just add your faster connection to the LB pool multiple times and that will use it that much more often. So in my example above, I'd add my Cable to the LB pool three times.
In both cases, out of every four connections I make, three go over Cable, one over DSL.