Had you given me the full error text I would probably not have suggested changing the ram chip, although it is probably close to the end of its life.
So, you either have a faulty rom and a misbehaving card, or some bigger problem. You can find out which it is by changing the card's battery and then formatting the card – look and see if the card checksum message is still present at powerup.
If it is, you have either a faulty card or a major hardware problem. If not, you have a faulty system rom – you could try and order one, but they are probably unavailable.
Either way, I'd suggest living with the message – seems like a small annoyance, and fixing it may be more difficult or expensive than it is worth. Remember, this unit is well over 10 years old by now.