Sapphire has quietly been working on a 1GB GDDR4 version of its Radeon HD 2900 Pro and although the core clocks are the same as the 512MB version, i.e. 600MHz, it looks like they've upped the memory clocks significantly.
We're talking a 250MHz boost from 1,600MHz for the 512MB version to 1,850MHz for the 1GB version. This is even 200MHz faster than the Radeon HD 2900 XT which has its memory clocked at 1,650MHz.
This might result in some better performance, altough we're not sure how much of an impact this has on real world performace. It could help in some texture intensive games, but the slower GPU clock might be a limiting factor here.
Our guess is that Sapphire is using 1GB Radeon HD 2900 XT cards that didn't qualify for hte higher clock speed, but this might be an entierly different product SKU.
You can check out Sapphire's Radeon HD 2900 Pro product page here and click on the spec's link to find the full specifications of the new card.