|"is there going to be a problem with XP being the Slave originally?"|
Excellent question!!! Yes, there is going to be a problem.
An operating system drive cannot have its address changed, and still work. Your XP Home disk has probably been installed as D: (the 40gb is C:). If you shuffle your drives around like you intend, the XP Home will end up as the C: drive, which is not going to work. It will also not have the boot files, which are on the 40gb drive.
I think you should byte the bullet and do a fresh install on the new drive.