Well, tonight he gave it a whirl with the "miniscule" 512 Mb and at the very outset, it lagged a little playing his online game, but that stopped quickly and didn't crash once during a 30-45 min run.
I saw on Newegg's site one of the customers *CHEERING* that his stick of DDR-333 Kingmax 512 Mb was read by his mobo as DDR-400; I guess if you have two modules of these, then it IS cause for a little joy... but if you mix'n'match, then, well, your kid cries, kvetches, moans, and otherwise makes known his unhappiness.
Anyhow, I am SINCERELY HOPING that this replacement stick I am to receive will behave exactly as a DDR-333 module should behave.