Sorry, this is a bit old now but just happened to be paging through.
Had to read the original post twice, but can't see the prob. as all SQ is asking is when a 512mb memory card is installed why does it show up as 128mb.
Bearing in mind the motherboard and chipset primarily determine the ram used, my first port of call re anything to do with memory is the Crucial System Scanner and the Crucial Memory Advisor tools.
http://www.crucial.com/
The scanner will tell you what it thinks you've got, and the advisor will tell you what you can use.
Give those a whirl first. The results may answer your question for you.