>Thanks for your patience, the reason I don't want them in RAID 5 at the moment , is, I have two 300gb SAS in at present but eventually I want four 1tb drives in RAID 5 so my thinking is ( probably flawed ) the machine works perfectly and obviously the OS is installed on the existing volume, but for the time being I need some more storage space without disturbing the current setup too much - am I heading down the wrong path ?

In theory rebuilding later works flawlessly. In practice heed Murphy's Laws ;-)
Is the time rebuilding/testing/checking/worrying worth the rather small amount of money saved by buying the 2 currently missing discs in a year compared to todays prices ? Probably not if just ***one*** thing goes wrong. So I would advise you to build the 4*1GB-drive Raid 5 NOW, move the OS from the 2 300GB to the raid and run from RAID and use the RAIDs added storage after a week of no problems. Take the 2 300GB disks and do yourself or a coworker a favor by giving another machine faster discs/a raid and enjoy not having to worry about HW for 18 month...