Ya, but considering that GTX580s are $500 each, dropping a pair in SLI would be expensive!
You should see significant improvements by going from 2x8600 to a GTX580. You might want to consider stepping up to a GTX460 or similar because the 580s will still be priced pretty high. I am assuming you are upgrading at the end of this year (2010)!
Ill push to 3.6 GHz and then probably end up waiting it out until AMD's 6900 series launches to see if nvidia sneaks in a quick price drop for competition. If they don't Ill probably still end up getting the 580 yet Ill keep the 460 in mind because they'll both be huge performance upgrades either way =)