your rig should be a good gamer, running most thing a high settings. the only weak spot would be the aging HD 6950 GPU. that would be the only part I could see upgrading, and only if you are playing with maxed out AF/AA. a GTX 670 or a HD 7870 would be good choices. you could also put in a 128-256GB SSD
I go with getting an SSD.
Video card you need to jump at least 2 tiers to notice much improvement, and that would put you into some pretty expensive cards, that in the end, wouldn't be all that much better over what you have now. You would be spending a lot for little return.