I've tried it with a probably functional Promise supertrak ex8350 in the pcie x16 slot and the thing just reboots itself over and over. I've heard of people using the newer PCIe raid cards in the x16 slots successfully on some boards including Asus ones. Search through the forums at storagereview.com.
Unfortunately it seems like the x16 slots might be designed by desktop motherboard makers with only video cards in mind.
With that said I'm using the a8n-vm csm with the onboard sata and some cheap silicon image pci (and soon pcie) controllers and doing linux software raid with this board with no issues. Granted the performance may not be as good as a hardware raid card solely on the PCIe bus but it's good enough for home use.
This board with 6 drives, onboard everything and an A64 3200 uses a bit over 110W idle with cool n'quiet enabled whereas my old system (Athlon 1700+, raid card, add on video, gigE NIC, and 1394 cards) with 6 older drives used about 150W idle.
Gaidin