It all dpends. If you a hardware RAID card that linux supports you should be able to just install into that last partition without any problems. Let me clairify a little before I go on. If you have a motherboard that has a RAID controller "built in" like a board with nvraid or similar, you do not have a hardware RAID controller. If you spent hundreds of dollars on an add-in card such as a 3Ware, LSI, or Adaptec, then you probably do have a "true" hardware RAID controller. Ok, now, again, if you have a real hardware RAID card, and linux supports it, you should be able to pop in your favorite distro's boot/install CD and go to town. If you have a "fake" hardware RAID card like nvidia's nvraid, or a cheap ass silicon image or similar highpoint card, you don't really have a hardware controller. What this means is that windows is doing all the RAID work in software, and I don't think it likes to play nice with Linux. I wouldn't want to mess with that silutation if you value your data. If you are not using any sort of RAID controller at all then that means you are using windows to do software RAID across those drives. If this is the current setup, again. I wouldn't want to touch those drives with linux. I don't think windows software RAID plays well with linux.
So I see three cases:
Real, honest to god, hardware RAID controller -> Install away!
"Fake" hardware controller (nvraid, similar) -> Don't risk it
Windows software RAID -> Don't risk it
Hope that helps!!