How To Tell if a Laptop RAM is Compatible and How to Use them ?

[slickcondo]

I hope this is the correct place to post this thread. If not, I trust it will be moved to a more appropriate place ?

I am hoping to get some guide and clarifications on how to tell if a laptop RAM is suitable/compatible for a laptop. For example, I have laptop A that is installed with a RAM of
512MB DDR PC2100 266MHz 200-pin SODIMM

I also have another laptop B that is no longer working, but has a working RAM of
512MB DDR PC2700 333MHz 200-pin SODIMM

Can the RAM from laptop B be used for laptop A ?

By just looking at the specifications of these 2 RAM, which factors should I look out for to determine if these two RAM are suitable and/or interchangeable between these 2 laptops - assuming both are laptops are working ?

To start with, I know that for both laptops,
a) the number of pins must be the same
b) the type, ie SODIMM must both be SODIMM
c) both must be the same DDR or DDR2 or DDR3, etc

But I am not sure if
i) the PC numbers (what does this stand for ?) must be the same
ii) the MHz numbers (I understand this signifies the speed) should be the same and
iii) are any other factors that I should be looking out for before using a RAM ?

Finally, about using these RAM, if a laptop specs says the Max RAM the laptop can take is say, 2GB and has 2 slots,
1) can a 2GB RAM be used in one slot only or must they be split up equally into 2 slots AND
2) does it need to be of EQUAL rating on each slot - ie 1 GB on each of the slots or
3) can 1 slot be used for !GB and he other for 512MB if the total 2GB is not required ?

Any help and clarifications would be greatly welcome and appreciated. Many thanks in advance.