What hardware for PCI passthrough?

...I have no experience that this will work with KVM and ProxMox in particular,
but what you'll need is a motherboard AND a CPU with support for virtual-I/O (i.e Intel vt-d).
Under VMWare ESXi, the feature "vmdirectpath" relies on it as well. There's a HCL available for this product.
I personally have good experience with Supermicro motherboards from the XEON class (like the X8SIL).
My "green" server runs that mobo with a L3426 CPU and vt-d successfully.
 
Chipset, CPU and BIOS all have to support hardware I/O virtualization.
"Intel VT-d" is what to look for in Intel stuff. AMD call it "AMD-vi" and also sometimes "AMD I/O virtualisation". The generic name is IOMMU.

Watch out when buying if you're getting consumer desktop grade gear - there's quite a lot of variation from manufacturers as to whether BIOSes support it, and there's also a few variations on chipsets with similar names where one model supports it but another doesn't.