Wow! So far, so good. I just got my GPU today and I've been at Tomb Raider and Bioshock Infinite for hours at maximum settings. I am not a fan of FPS games, and the only one I ever played start to finish was Turok 64, but Bioshock is very entertaining. I thought I would enjoy Tomb Raider more, but I was wrong.
StarCraft II sees absolutely no improvement, which I expected. I am still waiting on the 5160's to deliver. Diablo III is noticeably smoother, but it still streams a lot of data, presumably from the HDD or from the Internet, which creates a lot of periodic lag.
Speaking of the 5160's, before I bought them I did confirm they would work. Also, I have found this rig, which is precisely what I have, including the same exact motherboard, sporting two 5160's. That makes me feel great about the small investment:
http://stuartconnections.com/products/C ... ion18.html
Thank you guys for your great advice! I can't wait to play FC3: Blood Dragon. That game I look forward to the most.
So, I guess to answer my own question, my CPU does not plateau my GPU (or does not appear to) except in Starcraft II (which I didn't have hopes for, anyway, but thanks to your guys' recommendations I am going to improve that big time).