How would I know if a RAM chip is compatible?

Well, I'm thinking of buying a laptop with 1GB RAM and then buying another 1GB RAM chip myself and installing it myself. My question is - how would I know if the chip I install can function in the system? How do I know if it will even fit in the laptop? How will I know if its speed's difference (if there is any) with the preinstalled chip will affect the system or not?
Sorry if this has been asked before.

 

Most new notebooks use DDR2 SODIMM RAM, so when choosing the RAM you want, you need to narrow your search to just that type. Most notebooks will work with any brand of RAM installed. Some good brands are Crucial, Kingston, OCZ, Corsair and G. Skill. There are many others that are also very good. If you get RAM from any of those manufacturers, its highly likely that it will work with you notebook. The two frequencies used in DDR2 SODIMM RAM are 533 MHz and 667 MHz. When you purchase your notebook, you simply check to see which speed you have and buy the same frequency as that, in simple terms.

 

Also, most laptops when ordered will come with 2x512MB chips if you order 1GB of RAM. So you'd need to purchase both 1GB sticks to get 2GB.

 

Also, will Mac RAM chips work on windows laptops? Because a friend gave me a few Mac RAM chips, and I have no use.

 

Look at the specs of the laptop you are planning to buy, it should tell you the type and configuation (2x512mb or 1x1gb) of the installed memory. If that info is not availible, once you recieve your laptop, d/l CPU-Z and it will tell you the type and amount of memory onboard. Like night_2004 posted, you are probabely looking at purcasing 2ea 1gb sticks since most laptops come with 2x512mb sticks.