Replace HDD on computer

I've got a Dell Vostro equipped with a 1Tb RAID drive split in to 2 x 500Gb per the attached spec and am running out of space. I'd like to replace the 1Tb drive with 2 x 1.5Tb utilising RAID but so that the data is on two separate physical drives this time. This work needs to be done either overnight or at a weekend (the Diamond Jubilee weekend is also an option) in order to avoid downtime and should include data transfer from the current drive to the new so that we can switch off one night and restart the following working day morning. The work will be handled offsite with the machine being collected from and returned to London EC1.