Here's what I did, and results:
Bought a matching 2Gb DDR2 So-DIMM, and added it. They match, except for the product codes. (Not sure what the difference is, if any.)
So I've got:
Slot 1: Samsung M470T5663FBC-CF7 PC2-6400S-666-12-E3 (new one)
Slot 2: Samsung M470T5663QZ3-CF7 PC2-6400S-666-12-E3 (old one)
Both have 6-6-6-18 timing.
To find the product code of my original So-DIMM, I had to pull it out and flip it over. It was in slot 2. Replaced it carefully and (I thought) fully. Win 7 came up fine, and G. Chrome worked, and didn't notice any other problems.
It turned out that it wasn't seated properly. A week later, when starting MS Word, I got a BSoD: BAD_POOL_HEADER.
The cause was mysterious
, since I'd done a re-partition and a defrag in that week. However, I re-seated the old So-DIMM, installed the new one, and the BSoDs are gone.
Since I'm using 32-bit Win7, I only see 2.96 Gb of RAM, but oh, well.
Samsung claims their memory runs on lower power, so maybe I'll have better battery life, and since they're both Samsung and timed identically, maybe it'll run better. It was cheaper than replacing with a pair of 5-5-5-15 memory modules, uses better quality memory, and we're $20 further from starving.