I was originally going to 'resize' the two logical parititions like I've done before with Partition Magic on my home box but then I remembered the OS won't even see the extra space on my 73GB RAID 1 pair as the PERC doesn't tell it that it even exists.
Ouch. Was going to suggest just trying the same method that I know works on all Compaq Smart Array controllers until you said that. With the Smart Array, you can "fail" each drive in the array, and replace it with a larger drive. Once the array is back to normal status, the config program will show a chunk of free space in the defined array, and allow it to either be used to create a second logical drive (a second hard drive to the OS), or to extend the existing logical drive (assuming the OS on the server can support it).
Double check with Dell first to see if this is possible, (as a search on google isn't turning up much for me). If not, the other thing to ensure is that your OS won't be harmed if you later delete the 18gb RAID set after migrating to the 73GB drives. Some controllers may shift around the "virtual location" of the hard drives it presents to the OS, and that could cause the need for recovery.
Good luck. Keep us posted on this, as I'm always interested in how server hardware works from various vendors.