Well, some nice bad information in this thread. First of all, if you get a 6990 then you will be running tri fire since the 6990 is 2 6970s on a single board. Ref for those that are confused http://www.hardocp.com/article/2011/04/11/amd_radeon_69906970_crossfirex_trifire_review/
Honestly man, I wouldn't spend money just yet. See what happens with the 7850/70 pricing and see how the Nvidia Price/performance looks. I am in a similar boat, running 2 6970s and waiting to see what ends being the best upgrade if any for now.