R520, Radeon X1800 can address 1024MB memory
 

THERE IS a lot more that needs to be told about the R520 memory controller. ATI invested a lot of time and money to make it happen. When ATI was making the R520, it wanted to make at least one part that will last more than just one generation.

We asked around and it turns out that the R520 memory controller is very flexible, and can support even future memory standards such as GDDR4 but more interesting is the fact that it will let you use more than 512MB of memory.

http://www.theinquirer.net/?article=27336