Hi, I'm about to build a low-cost rig for GPGPU usage. I wonder, if motherboards with Intel P35 chipset (which has PCIe 1.1 bus) are able to host the newest PCIe 2.1 GPUs? For example: ASUS 6950 DCII gpu + ASUS P5K motherboard. Compatible?
They will work, just with some small performance losses. For GPGPU you should hardly notice it. Just be careful nothing you're running is CPU-bound, if you've an old slow Core 2 Duo or Pentium D in there, you'll very easily bottleneck such a powerful graphics card.
For PCIe x1, yes it will. How much depends on the GPU, and the task, but you may see 20-40% drops in performance using that interface.
The Asus P5K seems to have 2 x PCIe x16 ports (blue @ x16 mode, black @ x4 or x1 mode). How do you determine whether it's using x4 or x1 mode?
As far as I can tell that just means it is designed to be used with 4x or 1x cards, and it will run at 4x whenever you put a 16x card in there. If it is genuinely 1x, then that's obviously even worse.