RAM question....

OK...I'm thinking of upgrading my inadequate amount of RAM, but have a question to make sure I buy the right stuff 
I'm currently running 2x 512Mb DDR 200MHz(PC3200) CL3 sticks in dual channel slots (Making them 400MHz), and have two more slots to play with. Would it be OK to put in say, two 1024Mb sticks in these two, or would I need to replace the 512 ones with them. (Basically, can two different RAM sizes be run at the same time in two different dual channels?)
Also could anyone reccomend some good sticks?
Cheers,
.mic

I'm currently running 2x 512Mb DDR 200MHz(PC3200) CL3 sticks in dual channel slots (Making them 400MHz), and have two more slots to play with. Would it be OK to put in say, two 1024Mb sticks in these two, or would I need to replace the 512 ones with them. (Basically, can two different RAM sizes be run at the same time in two different dual channels?)
Also could anyone reccomend some good sticks?
Cheers,
.mic