Upgrade just the gpu?

right now i am rocking the bitfenix prodigy for a Lan party gaming rig. Love everything about my setup except that I only have the gtx 660 and some games i have to turn the resolution down or the settings. Would it be better for me to get a new motherboard and case for the option of sli and getting more preformance or should I just buy a more expensive gpu and if so should I sell by gtx 660?

Go with the better GPU. Sell your old one if you wish.

what are your specs

i5 3570k

16gb ram 

2tb

gigabyte z77n-wifi

bitfenix prodigy 

gtx 660

Definitely sell the 660 (only if you can get a decent amount back) and pick up a 760 or 770.  Nearly same specs as you and my 770 maxs ALL games at 1080p and maxs MOST(only 1 it doesnt max is Crysis 3 :c) games at 1440p.