There seem to be two versions of the Radeon X2800 XTX: one for OEM/SIs and one for the retail market. Both have 1GB DDR4 memory and while the OEM version is 12.4" long the retail version is 'only' 9.5" long.
The 12" OEM version has a power consumption of 270W and the retail version consumes 240W.
The slightly cheaper Radeon X2800 XT will be 9.5" long, have 512MB GDDR3 memory and will have a power consumption of 240W.
A cheaper Radeon X2800 XL is also planned but details of this version are still unknown.
This is the picture of the large OEM version, it looks like it has some kind of plastic thing at the end of the card to mount it to your case. I'm not sure if it's the same but I know that in the past large computer cards had similar plastic things at the end to mount them to a card guide in the front of your case. Few cases still have this nowadays.