Sony Vaio RAM Problems

I have a Sony Vaio PCG-FRV26 notebook and I'm trying to expand it's RAM. On the "Marketing Specifications" document (availabe at http://esupport.sony.com/US/perl/m [...] =PCGFRV26) it says that it's expansible to 1024MB of RAM. So what I did is buy 512MB and install it, taking off one of the 256MB and having in total 768MB of RAM (not counting what the video board uses).
But as I turn the notebook on, it will only show 512MB (RAM + Video) and not the 768MB.
I tried the following:
1) 1 256MB + 1 512MB = 512MB
2) 1 512MB = 256MB (!)
So, as I put only the 512MB chip, the computer says I have only 512MB.
Can the memory be "broken"?
Or maybe the notebook doesn't like the brand? It's a KByte 512MB DDR PC333/266.

Thanks for the help!