The card doesn't have to get the power through the slot, it will get it through the dedicated power connectors. If you really want to you can buy the cable from cablesaurus that injects power using a molex connector into the pci express extension cable.
First, it probably isnt worth paying additional 50 euro per slot + riser. If you decide to go that route you have to use powered extenders (~$22 at cablesaurus, around half of that done by some forum member I think, $3-4 if bought in bulk from China). But even then I wouldn't bother, especially if those mobos don't have an 8-pin EPS12V connector. They aren't designed to supply so much power through all pcie slots and you risk frying you motherboard, maybe even the GPU(s) can get hurt. While using a powered riser definitely decreases the risk it makes it even more unprofitable. I wouldn't bother unless I could buy that adapter and a powered extender for $20-30 in total.
Okay, these replies are confusing me. Phorensic's implies that I do NOT need a powered extender (if the card need not get power through the slot, it need not get power through a powered extender either, surely?). Then malevolent says 'you have to use powered extenders'. Well, which one of you is right? Does my extender need to be powered or not?
If I DO get a powered extender, I'm not quite sure why malevolent says it risks frying my motherboard. Powered extender means no power from the mobo, right? I connect it directly to the PSU with one of its 4-pin molex cables for peripherals, don't I?
To clarify, I am only doing this for a SINGLE PCI slot. The adapter could be either PCIe x1 or x16 size... I have since seen cheaper adapters than that 50 euro one, but they only offer x1 size. I would prefer the x16, IF the extender need not be powered AND the motherboard will be safe, because I already have an unpowered x16 extender. OTOH if I need to get a powered extender, since I'll have to buy that separately I may as well make it a x1 to x16 extender and buy one of the cheaper x1 adapters.