You can create a second array with the other 2 disks and install XP on this array, this will give you ~72gb for each array and will operate independantly from one another.
Use the "OCE" feature in the Adaptec RAID manager. This will increase the array to 4x36gb RAID-0 giving you your 144Gb disk. In windows you will still see your orignal partition and then a further ~72gb unpartitioned. This unpartitioned space can then be formatted and used for the XP install. Although i have always been told to install the older OS first, as XP could cause issues with your current Vista boot.
Then youll need some partition software (Acronis is good) Increase the size of your VISTA partition so that you have your 45gb left, then create a new partition of the remaining 45gb for the XP install.