No it definitely can use raid cards in the slots, I've got a highpoint rocketraid 4320 in there right now and it works just fine.
I'll try that dell bios thing though. Then again, if I can't boot from the Perc I have no reason to use it.
You can try the pin mod (check the OP), although if your board is ok with the PERC in the x4 slot it would be odd if it has SMBus problems with the x16/8 slots.
If that fails, maybe try a few different versions of mobo BIOS & see if you can boot it?