Your cpu will hold it back a bit at 1080p. In this video you can see a 965 holding back a gtx 1080 quite a bit in rottr.

In a review by TPU, the standard GTX 1080 takes about a 1% to maybe a 2% or so performance hit in a PCI-E 2.0 slot. So it would be a small hit but nothing to freak out about. A standard GTX 1080, 1070ti, or even the 1070 is more than enough for 1080p, I can't imagine what a 1080ti would do at 1080p, unless you were using it to drive a 144hz monitor. TBH, I wouldn't try to drive a 144hz monitor with a Socket 1366 CPU.
 
I have the x58 FTW3 board that I bought off some guy who was planning to build around it. The reason I bought it is because I needed a home for my Xeon x5690, and with the BIOS updated the Xeon works with the FTW3 board. You can find a Xeon w3680 or x5680 for around $65 on ebay. I paid a little more for the x5690 but I got it to 4.4ghz with almost no effort, but I remember struggling to get a 930 to 4.1ghz. The w3680 and w3690 have unlocked multipliers in case that interests you. These are the Xeon versions of the Core i7-980x and 990x, and they are great overclockers. 
 
Believe it or not, you don't need as much processing power at higher resolutions, but at the same time you don't want to pair a Pentium with a GTX 1080ti to drive a 4k monitor. At 4k you wouldn't see much of a difference between a Core i7-8700k and a Ryzen 5 1600x. However, the Core i7-8700k would knock off the Ryzen 5 or even the Ryzen 7 processors if you're trying to drive your FPS up while gaming on a 144hz monitor.

Show us some 3DMARK 11 and Fire Strike benchies when you can.