the 4GB GDDR5 AMD Radeon HD 6990 card isn't listed (as I'm customizing on the site) as being Crossfire enabled, while all 3 other options are. However the others are 2GB (GDDR5 or otherwise) so I'm having trouble keeping up with the new lingo and comparing.

Dual Video Cards
Dual 2GB GDDR5 AMD Radeon™ HD 5970 CrossfireX™ Enabled
Dual 2GB AMDRadeon™ HD 6950 - AMD CrossFireX™ Enabled

Others
Dual 4GB GDDR5 AMD Radeon™ HD 6990
Tri-CrossFireX 2GB GDDR5 AMD Radeon™ HD 6950