think of it this was one DDR333= 333MHZ two DDR333= 666MHZ, what dual ddr is puttin ddr in pairs to achieve a higher speed, they did this in the old days with SIMM where you had to install two at a time. But thanks to the o'le fellers at ASUS they made it so that you can install only one ddr, until you buy the second the mobo book will have all the details about the dual channel ddr in it. But for Gaming purposes nothing beats Dual ddr, not even RAMBUS, in fact dual ddr beats the crap outta rambus.
Yup, it'll get recognized as 512MB. The difference is when you have different size sticks and run dual ddr. If you have a 512MB stick and a 256MB one, and run dual ddr, you'll only see 512MB available (instead of the usual 768MB) since the 256MB is only being used from the 512MB stick.