Adding a stick of Ram - Help

Hello,

I have a couple questions here. I have two different computers that need more RAM. I went to crucial.com and it said that both computers require me to install matching pairs. -Now one computer- has 4 open slots with 2 already filled with 2X 512mb. -The second computer- has 4 open slots with 1 already filled 256mb.

Now my questions are can I just add 1GB of RAM to each computer?

Would it be better to remove the 256mb RAM and add 1GB?

Would it be better to add one 512mb RAM to the first computer?

also, the first computer is DDR ram and the second is DDR2 ram.

Help plz.

Thanx