Gasp!
I'm not surprised you are getting "insufficient memory" messages if you are processing raw files on a machine with only 256MB of RAM.
There are many different types of memory available and you need to get some that is compatible with your motherboard. There's more to it than hte number of pins, there's different types and speeds of memory that have the same number of pins. Depending on the make and model of your machine, the shop may be able to tell what kind you need, but don't be surprised if they need to take a look at it.
As for the "add or replace" issue, it all depends on how many memory slots your PC has, and how many of those have been filled by the memory you have fitted. If all your slots are full, you will have to throw away what you have to fit some bigger stuff. If you have spare slots, you may be able to fit more memory in the unused slots.
The good news is that, if you have free, usable slots, and you can buy the appropriate type of bigger memory, it is a doddle to add new memory modules.
I suggest you post some further information on what kind of PC you have, ideally listing the brand and model of motherboard you have fitted. A photo of the inside of your PC showing the memory slots would help. There are software tools, SiSoftSandra springs to mind, that can diagnose the type of motherboard/memory you have that will help you pick the correct replacement.
With a bit of luck, nirofo will be along to offer some further advice, he seems to be our expert in this area.
Regards,
Duncan