Adding 2 gig (2x1gig) Memory to PC with 2x 512

Hello, i have a question pertaining to what order ram should be installed when upgrading.

Im interested in buying.. [url=http://www.newegg.com/Product/Product.aspx?Item=N82E16820231098][/url] which would add 2 gigs of memory to my system.

Im currently on a Gateway gt5220. It has 2 512 sticks(pc4200) and has 4 slots. CPUZ says it supports dual channel.

So since dual channel is supported i should leave the 2x512's together and add the 2x1gig's in the 3rd and 4th slot? Or do i alternate when putting them in... Like (1) 512 (2) 1gig (3) 512 (4) 1gig

On a side note, with the newer ram being pc6400 it would clock down to match the old one.. would i rather just have the 2 new 1 gig sticks in and take the old 512 ones out or would the 3 gigs be better at the lower clock speed?

Thx for any assistance

Since this wasnt a custom build i didnt get specific paperwork on hardware, and i cant even remember what i did with the stuff that came with the pc when i got it.

Although throughout my searches online, i have read numerous reviews stating that users have upgraded their gt5220 to 3gigs or 1.5 etc.. Would there be any reason why i cant add 2 identical modules to the 2 already identical ones running dual channel?