well obviously you should not add a 300GB drive to an existing 38GB RAID 1(E) array, nor should you wipe it all and start over to create a mismatched raid 5....just silly....A backup would be a good idea anyhow, 38GB is not that much to burn to DVD's....
edit: Oops...you back up you data nightly...missed that part....
Your original idea of a secondary non RAID HDD for the 300GB is really the only option. Is the 300GB connected to the on-board SATA, or the adaptec card ? I'm not real up to speed on adaptecs, but I know some LSI's I have force you to create a 1 drive RAID 0 before it's usable....Dosen't make much sense I know, but it works. After you create the 1 drive RAID 0, the normal INIT procedure can then run, making it presentable to the OS. It would probablly be easier to connect it to the on-board SATA ( if you have it ) and chage the BIOS settings to non-raid. Watch your boot order too, as the on-board will usually have 1st preference by default before add-in cards.
I know the 15K's are fast....I have 28 of them on SCSI U320, but I've retired most of them now....I still use a couple for OS drives though....not much other use for them now-a-days...6U of rack space for 500GB of lightning fast RAID 10 is nice until you get the power bill....LOL