I recently bought a BFG GeForce 8800 GTS 512mb. My motherboard has a PCIe x16 slot; however, it is placed in such an awkward position that it does not utilize the full space of my pc case. In other words, due to bad foresight by me, and bad architecture and assembly by Dell and co., my video card cannot fit into my case while plugged into the only x16 port present.
I have plenty of space near the bottom of my case - all I need is some sort of PCIe x16 extender cable to help shift the video card 2 cm downwards.
Such a riser will work, but how are you going to mount it and your card to keep everything solid. In normal use this would be on the bottom with the card sitting on it and a support frame in the case.
If you do not mind a little modding(most finding a way to hold your video card solid since you can not just move it down as the card will not line up with the case pre drilled holes anymore and there will be other slots in the way preventing it from lining up) there is no reason it should not work(but its a expensive thing to experiment with). There should not be too much if any data speed decrease from the cable.
If you have a X8 slot you may be able to use that if it is open ended to use the card. There will be a performance hit, but you may or may not notice it.