Excellent, thanks guys. Already running an i5-2400 at 3.4 Ghz =) (presently running a single 6950 and it chokes on CryEngine2 at 5760x1200 unless I set the detail level to medium or lower so 3 will be worse I'm sure).
Now I just need to find a GTX-590.. but not until the 680 becomes readily available and drives the price down a bit!
I know I originally asked about the 6990, but looking at benchmarks in Crysis 2 (the engine I'm most interested in) it looks like the 590 does a little better. It does seem a little harder to find but I can lurk on ebay and watch for one.
Any reason not to do the 590 instead of the 6990? Again, CryEngine 3 is what I most care about.
If you happen to have a crossfire motherboard, and your 6950 is the 2gb version, you could purchase a 2nd 6950, and mod them both to 6970's (guides on this are abundant). That would be the same as a 6990 except with 2gb (effectively) of RAM.