Computer not recognizing new RAM

[mdob]

I'm not much of a computer person, so I could easily be missing something obvious here....
I recently decided to add some RAM to my computer. I had two 128MB cards and no open slots, for 256MB total. I did the scan at crucial.com and it reccommended a 256MB SDRAM card. I bought one on eBay. When I put it in with one of my old cards it said I still had only 256MB total both when I right-clicked my computer, properties and when I redid the crucial.com scan (it claimed 2 128MB cards, not 1 256MB card and 1 128 MB card). I then removed the old card and put the new card in by itself. I tried this in both slots and both times it told me I had only a 128MB card. Is there something I'm possibly doing wrong or did somebody on eBay try to cheat me by putting a 256MB sticker on a 128MB card?

Thanks for any help.