Definition of System Partition and Boot Partition - http://support.microsoft.com/kb/314470
The system and the boot volumes...are generally the same, as long as XP is installed on the C: partition. Installing XP on anything other than the first partition/drive installed...requires more thought than it merits.
If you are sure that you have XP now installed on the 30GB drive/partition...the smart thing would be to simply see if it will boot
without the other drive. That's what I would do. Simply (temporarily) disconnect the 15GB drive and replace it on the connector with the 30GB drive.
If the 30GB drive has XP properly installed, then I would simply add the 15GB file to the connector formerly occupied by the 30GB drive. To remove all files on the 15GB drive, I would then just format the drive, using Disk Management.
Disk Management - http://www.theeldergeek.com/disk_management.htm
If these are PATA drives (I'm sure they are), then you may also have to adjust the jumpers on each drive before connecting them in their new homes.http://www.hitachigst.com/hdd/support/cabling/deskstar4.htm
FWIW: Storage containers are hard drives, they are not RAM/memory. A 30GB hard drive provides 30GB worth of storage.
RAM/memory is something totally different. A typical system will have 1-4GB of RAM installed. The RAM does the actual computing work...before depositing files onto the hard drive for storage/future retrieval.