+1 for the tx 650m. partialy modualr (which is fine since the wires that are connected permanantly to the unit are cables you will have to use anyways). and its a solid psu, for a bit cheaper.
as for the 670 vs the 680. well its more like the 680 is 10% faster than the 670. but the 680 cost 27% more than the 670. why pay 27% more for 10% more performance. the ratio just dont make it value smart at all. the 670 i have runs everything no problem. everything is butter smooth, and if only i had a 120hz monitor, it would be even better because i know that this card can push out the frame rate needed for 120hz.
save $100 by going with the 670, and save a bit more by going with the corsair over the seasonic, and put it else where in your system, or save it. deffinatly go with the gigabyte ud3 or higher motherboard. with the money saved between the video card and psu, you could grab a ud5h, G1 sniper m3, or a ud4h, or up4.