[dachinesedude]

I have a Toshiba Satellite M300, comes with 2X1GB DDR2 667MHz, want to upgrade to 4GB so I bought 2 single 2GB Kingston DDR2 memories

Here's the thing, when I stick one Kingston ram into the notebook, it works, but when I stick both of them in, I get the blue screen. I tried out both Kingston rams and they work only if paired with a stock one.

I'm pretty noob with computer stuff, so was I suppose to buy the memory in pairs? The 2 singles arent working with each other, dunno if its the rule or just processor issue.

[InvisibleSoul]

Hmm... no, generally you don't need two identical modules. I think it's some anomoly with your laptop... maybe it can't handle 4GB of RAM.

[_Hotsauce_]

Check the manufactorers page about your Laptop and look for a RAM limit, Some are limited to 3g for some reason

[icemiko]

Check if the laptop can handle 4gb of ram and run memtest to check if there's errors on the new rams.

[syee]

Check if the laptop can handle 4gb of ram and run memtest to check if there's errors on the new rams.

Toshiba specs say max 4GB is supported (http://www.toshiba.ca/web/product.grp?section=1&group=223&product=8410)

Like icemiko said, run a memtest on them. Not sure if you tried this yet, but try each one of the modules individually in the laptop and see if any specific one fails to boot your machine.