running the 3870x2 is like running 2 video cards
If you had two 3870HD video cards, they would also only show up as having 512mb of memory - in lamens terms, this is just how it works when you use more than 1 video card, it doesn't get more ram.
2 video cores lets the other functions of the video card stack together - somtimes it is cost effective to add a second card over going to the next tier of graphics for the best performance if you already own a specific card.
A lot of this would take more writing to explain than I care to be bothered with and there is TONS and TONS of documentation out there which explains this in great detail already. Long story short, you need to do your own research and be a bit tech savvy or you're not going to be able to make an educated decision when you do buy graphics upgrades.
On a plus side, the 3870x2 is a pretty good video card in all truth; so you really didn't go wrong at least.