Actually I have 10,000 VBR's on my two 30's and I've got 6 GB left.
Your 300 CD's are only about 4200 songs, which would fit well under
an additional 30GB of disk space.

With Disk prices always falling,
you don't want to pay for disk space until you need it.

For examplee, my unit has two 30 GB disks.
I'll upgrade one of the disks to a 60 GB when my empeg is full and
I have an additional 30 GB of music to put on it.
No sense in buying a 60 GB disk prematurely just cause
I have a couple few extra GB that don't fit on the unit.

Then on the next upgrade I'll replace the other 30 GB with an 80GB
the upgrade after that, I'll replace the 60 with something else bigger,
so on and so fourth.
Unless my music collection get gets REALLY big in a short ammount of time,
I can't see needing to upgrade both hard drives at the same time.
Arguably, there's the "youve got the hood up, and you may as well" factor,
but swapping a hard disk isn't all that that hard.

This is my philosophy, anyway.
Obviously, you are free to do as you wish.