NEED :RAM Guide

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i have two 512 MB rams
I have 865GBf Intel board
Which is the best method to install additional RAM?
Current memorry settings
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Single channel
memory channel a slot o :512 Installed (DDR 400)
memory channel a slot 1 :none
memory channel b slot o :none
memory channel b slot 1 :none

Which slot should i fill?
I have another simmtronix 512mb ddr 400
Which RAM should i buy another 512 or 1 Gb?

This board supports dual channel so stick it in memory channel b slot o.

If you are getting new ram get 2gb of DDR. and put them in the a/b slots also.removing the 512 of course ;)