Robert,
For amplifying the output of a load cell I think you should use an instrumentation amplifier. I have use an INA114 in a situation like yours. You can buy a descrete chip or you can construct one with 3 op-amps an some resistors.
The fact that the output voltage goes down as the weight increases maybe fixed by swapping the excitation voltage or turning the load cell the other way up. If not it shouldn't be hard to fix it in the software.

Hope this helps.
Mike