There has been a few people trying to get bootable raid arrays working with the supermicro h8dce, since I have bought a few more hdds I have some drives to play around and came up with these pointers.
Also use this procedure a normal single drive boot system with a RAID array for storage/data, because unless you have the all the RAID funtionality enabled when you install windows you will not be able to create "nvidia raid arrays" in windows, BUT you could make normal windows software raid arrrays if you don't enable nvidia raid propelry in the bios.
In order to get the nvidia raid controller in the correct state you have do some settings in the bios.
1) set the P-ATA as the first boot devices...NOT S-ATA(gotcha number 1)
2) the enable nvidia raid rom, in the ide features screen(as well as both sata channels)
3) enable raid function on both master and slave sata devices.(ide/nvidia raid subscreen)
4) save settings and reboot, then press f10 when prompted to go to the the raid config screen and create your multidrive array, if you want to boot from a single drive create a single drive array. You can add rives to the array later if you need to anyway.
the following 3 parts may have to be gone through a few times to get all the setting in place, because you some items depend on others,(gotcha number 2)
5a)Once you have created the array you should get the option to make one of your arrays bootable, if the option is greyed out then do save your array and reboot and go back into the bios.
5b) go to the boot options menu and choose configure hard drive, you should be able to select NV array, if you haven't then some of the above settings have not been set.
Then select boot order, make sure the NV-array is in the boot order, just below cd/dvd rom should be the correct spot, save your settings and reboot.
5c) go back into the NV RAID screen(F10) and now you should be able to make the array bootable.
6) get your RAID driver floppy disk ready for your windows install.
7) boot you windows installer , selecting f6 when prompted and insert the driver disk and choose the raid driver when asked for scsior storage drivers.
8) from here on in it should be business as usual, with you array showing up as single drive.
once you have widows installed you should be able to add drives to your array nad build new ones(once you install th full nvidia nforce driver package.
Hope this helps some people
PS: Its pointless trying to install windows on your array, unless you raid array shows up as bootable in the NVIDIA_RAID configuration controller boot screen, and is selectable as NV-ARRAY in the bios boot order options.