OS version can be checked on the "Other" screen I think. If it doesn't say 16MB or 128MB Installed on the boot up screen, check the available recording time. If it says a little over 2 minutes for Mono it's only the stock 16MB. If it's a little over 24 minutes for Mono recording you have the 128MB RAM upgrade already installed.
I don't have a 500 so I don't know if it shows RAM on the boot screen or what screen you can check on. But looking at the available sampling time will answer the question too.
If you find out you need a 128MB find a cheap RAM stick matching these specs, CL3 (Clock Latency) being extra important. You can't 100% guarantee that a RAM stick with those specs will work, but every RAM stick I ever saw not work didn't have any CL # listed, let alone CL3. (The Akai RAM is the same thing with an Akai sticker on it for $150. These sticks are actually like $5-$20 tops online)
SDRAM PC133 256MB 144pin 133MHz SODIMM CL3
