break out your wallet, blow the dust out, and spend a few bucks on some RAM
Sometimes the hardware is so old, that it is not as easy as that to find the right RAM for it...
I have some very old IBM PoS cashier machines (sic! got them for 30 EUR each, plenty of connectors, very well built) based on Celeron processors which valiantly run CentOS 5 for some tests/router/storage, etc., but I have a hard time finding RAM for them. I think that they will never make it to CentOS 6 (maybe I'll switch them to Debian which I always found pretty good for low-end hardware).