Dual Channel memory configuration, requires that you configure the ram in two seperate memory channels as it uses two memory controllers to enable the Dual Channel memory mode.
You will need to buy two memory modules to achieve Dual Channel Mode, so if you plan on only running 512MB of DDR then you need to buy 2x256MB modules, if you plan on running 1GB which is what i recommend then you need to buy 2x512MB modules.
As a side note, I recommend that you buy pc3200 as this will help future proof you, and also allow some Overclocking.
Crucial is an excellent choice as it is the industry standard module.
<font color=blue> <b> <Capt Archer> Cant this thing go any faster, I thought this was a warp 5 engine.
<Trip> yeah, on paper! </b> </font color=blue>