The "Dell Diagnostic Partition" is a handy set of tools.
You can still restore it but it is not necessary, since the diagnostic tools can be started from the bootable cd you shoud have received with your Dell.
Let me stress the point again: if you do care for your data, you should at least have a decent backup.
A tested method - as sysadmin i regularly encounter problems with unreadable tapes and backups that have not run, etc etc.
As for storing data on a single disk... my motto :
Imagine, like a good friend of mine had to, your harddisk dies.
All data gone.
My buddy was sick to his stomach, literally.
I managed to recover some data...
but most of HIS data was recovered from an old backup of MINE....
from a time i worked on HIS system....
Point is.... you choose how important and recoverable your data is.
I even backup my buddy his data....
Maybe you have a good backup system, or choose not to have a good backup.
I cannot help stressing the point - an occupational idiocy i guess :-))
Have you checked the bios ?
If your new HDD is not properly supported by your bios, data corruption can happen.
If your bios does not 'see' the new HDD as 320 GB you probably have a problem, even though all seems fine.