You should get another stick of ddr3 so you can get dual channel memory performance. You also should up your cooling so you can overclock, because there is a high chance of the 955 bottlenecking the 680.
1. Get another stick of ddr3 ram of the same speed and size.
2. Up your cooling to something like a corsair H100 or swap out for 2 both high CFM fans.
3. Get a better powersupply.
4. You should have enough for a 7870 or 7850.
i agree with everything except cooler.H100 is $100+ still you need to put your own fan.it is very expensive and 'not worth it' solution.hyper 212 EVO is a great cooler.i wouldn't recommend changing.