Laptop RAM issues

I have a Toshiba Satellite A105 laptop that I'm working on. I have two 1 GB sticks of RAM for it, but one is PC2-5300 and the other is PC2-4200. Is there some reason the laptop won't use both at the same time? I'd been running the 5300 stick with a PC2-4200 256 MB stick and I had no issues with that pairing, and either stick functions fine on its own.

Hmm, does either stick function fine on its own in either slot?

No reason why they shouldn't work together, albeit running at the slowest (4200) speed.

Actually, upon further investigation, I found the following:

-The 5300 RAM won't run by itself, it will only run when paired with the 256MB 4200 stick. With just the 5300 RAM, the computer refuses to boot.
-The location of any of the sticks of RAM doesn't matter.
-With the 1GB 4200 stick and the 256MB stick, the computer recognizes 1GB of RAM.
-With the 1GB 4200 stick and the 1GB 5300 stick, the computer recognizes 1GB of RAM. I can't figure out which one it's using.
-With the 1GB 5300 stick and the 256MB stick, the computer recognizes 1.28GB of RAM.

I think the best thing to do would be to get another stick identical with either of the 1GB sticks, probably the 4200 one as I don't know what's going on with the 5300 one.

Any thoughts before I start ordering stuff?

it's always a good idea to have matching ram sticks. different motherboards will react differently to different ram sticks, and it's really impossible to tell what a particular board will tolerate. since memory is so cheap these days, it's often better to throw out your old sticks and order new ones on every update, or know exactly what you have and order a matching set.

like Moriz says - it's so cheap now that you really should get matching RAM DIMMs to aleviate the worrisome situations that you have been having.

RAM can be an absolute pig. I had a customer who had 3 identical machines (same motherboards, right down to the Bios revisions..), all running PC4200. Wanted them upgrading to 1 GB. 2 Boards would take the 1 GB sticks I bought, one just wouldn't play nice with 1GB sticks. Thankfully, it would recognise the 512 MB sticks from the other machines, so it got those instead. Saved by the "replace all the old RAM, interoperability can be an issue" policy.