I have currently 2 x 512 MB DDR RAM in my PC:

1 x 512 MB DDR PC2700
1 x 512 MB DDR PC3200

My mainbaord supports 400 MHz

Is it recommended to buy another 512 MB DDR PC3200 (for the PC2700)
to let both Ram module work at the same speed ?

Or is the current speed loss not so dramatic?