I am in the same situation as you, Core i7 950 (OCed to 4GHz though with a noctua cooler, 4.2GHz was a bit unstable after 2 years for some reason), 12 gigs of RAM (6 sticks of 2GB for that triple channel), Dual GTX 580s 1.5GB as well 3 SSDs. I still think the rig is phenomenal as far as 1080p gaming goes, but I've been wanting to upgrade either this year or the next to go 1440p or 4k, and I don't feel like putting another 2k on a rig, so I'd keep a GTX 580 as a Physx processor and put in a brand new card to give it a few more years of life until the next big thing arrives (DDR4, PCIE 4.0, etc).
I am wondering the same thing though about the CPU being a bottleneck for a new GPU, however, considering it used to be Intel's enthusiast socket (socket 1366 --> socket 2011) and it's a Core i7 and still performs very well, I am guessing it shouldn't be but then I have no way to confirm myself :S Don't think any game fully uses this CPU yet though, unless I'm mistaken?