Hard Drive help please

[drewz21]

Ok, I've got an A4000 with a 4GB IDE hard drive and an IDE CD-ROM drive.  It's got 3.1 ROMs and AmigaOS 3.5.

My question is how difficult would it be to add/change to a 20GB IDE drive or do I even need it?

In the Amiga world is 4GB enough for a classic machine?  
Is 20GB overkill?
Should I upgrade to AmigaOS 3.9 first?
Should I keep the 4GB drive as a system drive and then add the 20GB as a secondary drive?
Do I need some hardware in order to do that?

Thanks for all your help/advice.

[amigagr]

remove the cdrom, connect the 20gb as slave, partition and format it, copy all data from 4gb to 20gb, remove 4gb, put 20gb as master in it's place and put back cdrom as slave. you can install 3.9 later, after that you will have back up the boot partition for safe reasons. make one or two spare partitions on new disk for temporary back ups. :-)

-edit-
for me it's better to install the new disk with sfs and not ffs from the scratch sfs
it's safer and can handle easyer big partitions.think about it
keep the old disk for backup reasons too!