it seems to me a balanced machine.
any gpu upgrade is not going to make a noteworthy difference if you play @ 1440x or lower resolution on that machine.your budget is for another 4670 and a mobo....
assuming you have a generic psu... so adding a psu to the config too..
so say it would cost you around 10k for the transformation..
rather sell current 4670 and athlon+board for say 3-3.5k and 3.5-3.7k respectively..
17-18k budget...
good enough to include an athlon II X4, entry level 760/780G, a decent PSU and HD4850/GTS250... for now or maybe even the next gen HD57xx/56xx series card in 2-3months..