I thought you were replacing the ram. You're adding more. So your best bet is to get the exact same ram even if it costs a bit more. You're guaranteed a working pair. If you throw in the SLI ram, it will work after some tweaking in bios. Give it a bit more joice in vdimm. That'll be 4x1GB.
Ah, it's AM2. So overclocking the new ram will take some time. I'd still get the exact same.