Dell Poweredge 2650 Memory Upgrade Problems

Hi There all

I have PE2650 and it has got 2 512MB PC1600 ram  sticks in banks 1 and 2, I put in a 1GB PC2100 into bank 3 and boot up, getting memory incompatable or unconfigured message during post just after ESM info is displayed. It says press F1 to continue or F2 for setup. In setup it shows 1GB. If iI continue to boot wit shows 1GB in windows.

If I just put the 1GB stick in on its own it shows 1GB so the memory is ok.

I thought it would run together but at the lower speed!!

Do I need to run matched pairs in this machine?

Thanks in advance

Gavin