Assuming you're using XP, try using both. As you said, it should run as PC2700. Go into BIOS and see it the 2 modules are doing that. Then boot into Windows to see if there's any problem. If no problem, keep it that way. If you run into problems or instability, remove the old module and just use the new one.
You might even get the nForce2 dual channel memory that you were after originally. I'm not sure dual channel will kick in - usually they should be same speed and size, but with the new one being reduced to PC2700 anyway, it might just work.