You can, if you're using software RAID. However, software RAID is only supported with:
1) Windows 2000 Server
2) Windows Server 2003
3) Windows Server 2008
4) Dynamic Disks.
In that case, for example's sake:
Disk 1: 80 GB
Disk 2: 250 GB
80 GB of Disk 2 can be used as a software RAID towards Disk 1. The rest can be used however you want.
Linux may totally crap on dynamic disks...I don't know.
Barring that, dynamic disks are a pain. Also Ghost doesn't recognize software RAID (not sure about Acronis).
As for hardware RAID, you'll probably need a complex RAID controller to pull that off, and even then...good luck.