From everything I've read in the past here and on other sites I do not believe it will. Your benchmark scores may be lower because of CPU, RAM speeds vs newer tech but the card itself should be fine on a PCIe 2.0 @ 16x.
yeah, i'm mainly just looking to play older games also, but want to get the frame rates up to 120+ as much as possible on my 120hz monitor....i can barely stand 50fps or lower, and 60 just barely cuts it for me....120 is pretty awesome, when i'm able to do it.
mainly playing WoW, D3 & Skyrim w/mods, and my 6870 struggles in areas on all of them with all the settings turned up. not to mention, the micro-stutter in Skyrim is absolutely HORRIBLE. hopefully one of these new nVidia cards will let me max everything out and still get 120fps most or all the time.
You'll be 100% fine... I had the same concern when getting two GTX 970's to OC on my PCI-E 2.0 x8 per slot system (for SLI configurations) with an i7 2600K overclocked. Here's some benchmarking and testing by a system manufacturer using Titans:
Nice in-depth article, the takeaway is that in virtually every situation including 1080p and 4k resolutions you will see margin-of-error differences. In just one of the configured sub-tests at 4k they saw a whopping ~1fps difference (44 vs 45) in their results.