Thanks very very much for checking the part number and confirming the model and OS versions. You're a gem. I was worried about compatibility, but I feel a whole lot safer now. I'm stalking a couple of SODIMMs on eBay and will buy one soon. When I have it installed and tested, I'll post the results here. That will probably be a couple of weeks from now.
I only found one other report on the internet of anyone trying this. It was in an Amazon review from 2002 for a Sony VAIO R505GL, which, like the R505DL, Sony says can only handle 384 MB. The guy installed a 512 MB memory stick. He also used crucial memory (but he didn't give the part number). The guy reported that it was working fine "so far".
Don't worry - I won't blame you if it doesn't work. It's well worth the risk to have that much memory. Thanks again.