IMHO, SuperDrive is almost useless, except for those of us who work with video (it comes handy when you need to give someone a piece of footage to watch). Recordable optical media is faulty, and 2.5-inch USB enclosures let you back up your whole drive even on the go.
On the other hand, the faster your HDD, the lower are load times. I have a cheap 60GB SSD in my PowerBook — no miving parts, no noise, no worries. Don't forget to add
'''mount -uw -o noatime /'''
to your /etc/rc.local — it restricts changing flies' 'access-time' field only to write operations, thus improving drive's performance greatly and significantly prolonging drive's life.
Of course, additional RAM is a must. The more RAM, the better.
Also. There's a company selling a kit for installing secondary HDD instead of optical drive - Optidrive, AFAIR — check them out.