Hi! First I would reset the PMU and PRAM. If that doesn't help, I'd remove the extended RAM, boot up, and see what amount it says is installed. If it is correct (I believe it should be 256MB based on what you've mentioned), then install the extended RAM again, boot up, and see what the total is.
If none of this works, I'd boot from the Hardware Check CD and see if it reports any issues. Good luck!