I would have to agree with Brian on this, at least for a planted tank. I use a Magnum 350 with a micron cartridge on my 75g tank. I have used a Fluval 304 in conjunction with the Magnum for an increase in flow but have found the Magnum 350 works just fine, with our without bio media. I think in a planted tank the fish load is usually low enough that we really don't need biological filtration. The plants seem to take care of this on their own and Magnums are also excellent CO2 reactors
For fish only tanks, I have heard lots of people complain about the Magnums and their lack of room for biological media. I think this issue is more than covered with the Magnum Pro systems and their included bio-wheels. The addition of some bio media to the Magnum probably won't hurt, but I prefer to use them with floss or the micron cartridges to keep as much gunk as possible off of the bio wheels.
Mmarino,
If you want to plant the tank
and use CO2 you can either continue to use the biowheels and increase your CO2 input into the tank or remove them and go with a lesser amount of CO2.