Depends on the flexibility of the RAID BIOS. Short version: Probably not. You'll have to spend a good bit of time reading PDFs and the like to find a controller with this capability. Most consumer level controllers simply use the capacity of the smaller drive and waste the extra capacity on the larger one.
Running W2k3 on a duallie P3 750. I just was too dense to pick up 2 120gb maxtors when they were super cheap, so I'm trying to work with what I've got.