I have a couple of servers that only support PCIe x8 slots. Are there any GPUs out there with a PCIe x8 form factor (preferably low-profile, so I don't have to get a riser card) that would be worth messing with to function strictly as BOINC processors? The only thing I could find for an x8 slot was an adapter that allowed connecting an external array of GPUs that was ridiculously expensive.

One of these servers has an x16 physical slot that is limited to x8 speed due to a RAID adapter that quasi-shares the bus channel. I tried plugging in an nVidia x16 card, but it caused serious problems. My guess is that this consumer-grade card did not know how to reduce its expectations to work in what is effectively an x8 slot. Is there a way to evaluate x16 form factor GPUs for their ability to function when the channel is reduced to x8 speeds?

I believe every modern GPU card can work in x16 slot with 8x speed. I have one GTX 760 running PCIe 2.0 x4 at the moment and it's doing fine.

My guess is, the serious problems you mentioned, propably occurred because of the RAID adapter having problems in the system in that situation. Or something else. Not because the Nvidia card not being able to work with x8 in x16 slot.

What exact model was that Nvidia card? Which driver version did you install for it? What PCIe "version" does that motherboard support? 1.1 or 2.0... ?

Also other specs of the hardware would be helpful to know. RAM, CPU... and how much storage and other stuff is connected to the board?
And the operating system?

I have no doubt that the RAID controller is the source of the problem. It's an Asus PIKE card, which is a proprietary adaptation of an LSI 1068E chipset. I would love to replace it with something more normal, but I'm not in a position to do that right now. The motherboard is PCIe ver. 2.1 I think. I currently have a GTX 750Ti humming along smoothly. The GPU I tried was a 8800GTS that I found collecting dust on my shelf. When I put it in, the computer wouldn't even boot far enough to enter the BIOS settings.

The other machine I really want to give a GPU is an IBM x3650, which is in a 2U case and only has x8 slots.