Sapphire has the 3850 card with 1GB of memory. This card will cost only slightly more and there will be two designs: one with DDR2 and one with DDR3.
The DDR2 card should be ready in a week or two and it won't be much more expensive than the 512MB version.
If you put two of these together you are talking about Crossfire with 2GB of graphic memory.
This is how it looks.