That shouldn't matter. That stick of PC3200 will still run and just downclock to PC2700 speed. Did you try putting the 512 stick in ram slot #1 and the other one in #2? Maybe that will help?
If you can get it to work, you're better off with the PC3200 stick than if you had gotten another PC2700 because all new mobo's that use DDR memory now support PC3200, so if you do upgrade in the somewhat near future, you've already got a stick of RAM for the upgrade. If you were to try and stick an old PC2700 stick in a newer board, it would work, but your system RAM would only be running at PC2700 speeds instead of what the optimum that the new mobo can support (PC3200).