msi 790 fx-gd70 possible upgrades?

video card upgrades for this mobo. cpu ram and ssd/hdd are all obvious and easy choices. the difficulty I found was in video cards. I want to take advantage of the 4 slots my board offers but the newer xpensive cards seem to occupy slot width( x 2) I.e. I could at most use 2 cards. and if that is necessary I spose the best I could get is the gtx 690 . can u confirm this is the most economical upgrade from my twin radeon hd5670's?
 
the game im currently playing is f14 arr. at 1080p(1920x1080) or w/e it is. I run it at standard destop and it does ok but that is when I solo or small group/ empty area. in large groups ( ie massive particle generation) I drop to 12-15 fps and even the audio starts skipping.( tho an audio card instead of on board would prolly fix that) but solo just me n the game I get 55-62 fps on standard 30-35 on max w my current rig.
but everyone knows mmo's are meant for group play not solo. which is why I want to upgrade. tired of the fps/lag issues from having an outdated computer.