Check out this auction:

http://cgi.ebay.com/Macintosh-Portable-5120-(Non-Backlit)-PDS-2mb-RAM-Card_W0QQitemZ260313018127QQcmdZViewItemQQimsxZ20081109?IMSfp=TL081109104010r20966

I think these cards "stacked", so it's possible you just have a lower capacity model. System 6 can definitely handle up to 8 megs (my current Backlit Portable has 9MB and is running 6.0.8 ).