Ram question

[VoodooAxis]

I have a Asus P4P800 Deluxe mobo, I think I have 1 gig
of 3500 DDR that runs at 434Mhz.I only run my machine at
400 Mhz,so I would like to know if I could just buy a gig of
400 Mhz DDR ram to add to the other for 2 Ghz of DDR ram ?

Thanks,
VoodooAxis