minimal performance gain in gaming over 8Gb ram now, but no real gain having an i7 compared to an i5 either, as most games today won't benefit from the hyperthreading... stuff like that is more helpful for things like photoshop and video rendering. You could buy a second 7950, but if you can afford it, general rule I was always told was buy the best single gpu you can vs. 2 lesser ones. Although if you already have a 7950 and could only afford another one (the 250 to 300 dollar range), then to step up considerably from there would cost a lot more, like a 7970 or a 670/680/690 in the 500 to 1000 dollar range. So if you can afford it, why not
also, consider an SSD. I just got my first one I'd ever had since prices have come down so much and it's crazy how much quicker they are.
I've also heard of issues with microstutter on some games, although I'm sure your more recent titles are better at multi-gpu.
My rig pales in comparison to yours, i5 3570, 8Gb 1600 ram, 7850 2Gb OC, but for 800 bucks I'm extremely happy with it and hit 60+fps on almost all games I play in high/ultra settings (Arkham Asylum, Skyrim, GW2, etc). Your single 7950, as I'm sure you're aware is perfectly fine.
I guess it all depends on how much money you want to spend.