There seems to never be a definite answer concerning these, I want to know the definite differances. Ive heard say the 680s are way more stable and outperform the 650s, Ive heard others say they are identical except for the fact the 680s have 2 16x slots while the 650 only has 1 16x slot which means you can't SLI 2 geforce cards. Opinions please?