RAM explained...will I be able to UPGRADE from 512MB Dual Channel DDR2

[teddyFBI]

The new Dell I'm buying has the following listed for the base RAM configuration:

512MB Dual Channel DDR2 SDRAM at 533MHz - 2DIMMs

I have the option of upgrading to 1GB for another $100, but someone told me it would be FAR cheaper to buy my RAM separately from another retailer (I checked, and Newegg does indeed have 512 for $40).

I'm a bit of a tech donk...if I just buy an additional 512MB from Newegg, will I just be able to drop it into an open slot on my Dell??

I thought I remembered hearing a long time ago that if your RAM is "dual channel", then it takes up BOTH available slots, and so you won't be able to upgrade...or something like that...is there any truth to that?
Any other suggestions welcome!