New Configuration--Partition size

[MountaiNet Tech Support]

I am configuring a new machine for qmail.  The machine is a Dual 400 with
512 Meg of RAM.  Currently we will be using an 18gig drive (RAID 5 setup)
for e-mail only.  What would be the best way to partition this drive for
qmail to allow for queue and everything for optimal performance?  Also, if
later I add another drive to it increasing it to 27 gig, how hard will it
be to add this extra space to my /home dir for additional storage?  How big
of a swap file should I make?

Thanks