What's the highest-end GPU for PCI-e 2.0?

This is also a function of how many lanes your motherboard supports + how many lanes your graphics card supports.

Likely you can search Tom's Hardware + other tech forums for details about this.

Then look at ultra high end motherboards, in the $1K-$3K+ range + read about their lane setups.

Running a slow GPU with more lanes will likely outperform a fast GPU where your motherboard only support 1 or two lanes.

This is a complex topic. Likely you'll have to research it a good bit to make your choice.

It is indeed complex, take GPU crypto-currency mining for example. A single PCIe lane works just as well as a 16 lane card since it doesn't take much bandwidth to pass the instructions/data to the GPU but it takes the GPU one heck of a lot of power to process it. An analogy of that would be for me to ask you what the 1000th digit of Pi is, that'll take you several hours to work out and the answer is simply "8".

andyalder makes a good point + is correct.

1) If your gaming, better to have many lanes + a cheaper GPU. Or best many lanes + multi-GPU, with blazing fast GPUs.

2) For bitcoin mining, lanes are unimportant, as bandwidth is negligible + in this case the more + faster GPUs, the better.

Best you describe your specific application for best assistance.

>>  1) If your gaming, better to have many lanes + a cheaper GPU. Or best many lanes + multi-GPU, with blazing fast GPUs.

Question:  You are referring to the lanes on the PCI-Express slot aren't you?  Where the number of lanes could be anything from 2 to 32?

LOL Bitcoin mining?  I thought that was only possible if you're running a large server farm with teams of people.  Unless there's some new information out there on how a single person could make money.

No, I'm just looking to game as of now.

I said GPU based crypto-currency (such as Zcash), not Bitcoin; but it was just one example where GPU performance rather than PCIe bandwidth is the deciding factor. Anyone can make money mining Bitcoin with a single ASIC miner by the way, just point it to a pool of miners and take your slice of profits.

You might find it's a combination of factors - the video card drivers used, the CPU, RAM speed/latency, BIOS version in the motherboard, PCIe settings, Windows Motherboard driver etc.

Often, the CPU specs can limit graphics card performance - are the benchmarks you refer to using similar hardware to the rest of your PC?

When you installed the drivers, did you select the 'clean install' which should have completely removed the earlier drivers?

Check your power management settings to ensure not throttling is occurring - try switching from balanced to high performance?

You might also want to check whether the other 970s were stock devices or 3rd party enhanced versions with higher GPU and memory speeds.

You might find it's a combination of factors - the video card drivers used, the CPU, RAM speed/latency, BIOS version in the motherboard, PCIe settings, Windows Motherboard driver etc.

Often, the CPU specs can limit graphics card performance - are the benchmarks you refer to using similar hardware to the rest of your PC?

When you installed the drivers, did you select the 'clean install' which should have completely removed the earlier drivers?

Check your power management settings to ensure not throttling is occurring - try switching from balanced to high performance?

You might also want to check whether the other 970s were stock devices or 3rd party enhanced versions with higher GPU and memory speeds.

I looked under Power Options -> PCI Express -> Link State Power Management and it's turned to Off.  Should I turn it on?

No, I don't think I did a clean install.  Let me try reinstalling with that.