ATI will quietly introduce the R600XTX card with 1024 MB of memory, but this card will be an OEM baby only. There won't be any retailers selling this 12.4 inch card, only the big OEMs will get it. We wrote about these cards here.
This card might end up slightly faster than the Radeon HD 2900XT 512 MB DDR3, but the faster memory wont mean that much faster performance. The memory on the 1024 MB card works at 2.2 GHz.
You might notice that the card is also renamed to Radeon HD 2900XT 1024 DDR4 and that ATI didn't dare to use the XTX brand. This means that something faster is coming.