This is the 4th time I write this in Macrumors but, here we go.
When you create a partition, the computer reconize each partition as a different drive. That means that each partition would have a different level of fragmentation and if you get a virus it will go to one partition only in place of affecting the entire drive.
Another caracteristic is that the sectors would be smaller in each partition.
Now, I have a set up of partitions in my computer that have been very usefull. I have been doing this for at list 5 years now and it works great.
Now, in my case my hard drive has 20G and I'm using sys 9.2 only. As you are using OSX you may need to ad another partition. Wel, my HD is set up this way:
1st partition: 1GB just for the "Sys", Quicktime and OS related only, not even 3rd party tools.
2nd partition: 5GB "Soft", in here I had divided each program by categories:
Audio: Digital Performer, Protools, iTunes (the mp3 folder goes in to the next partition), etc.
Video&Animation: Final Cut Pro, After Effects, Miromotion, etc.
Publishing: Photoshop, Illustrator, M$ Office, Scanner stuff, etc.
Tools: Norton, Toaster, Stuffit Expander, etc.
Web: Dreamweaver, Flash, Hotline, MSN Messenger, Netscape, etc.
VJ: This is whre my VJ softwares are.
3rd partition: 15.4GB, here is where I drop everything, pictures, mp3, text files, downloads, video, my parents and other users folders, etc. Here I install the new software before officially apruved, this is to prevent undesire fragmentation in the "Soft" partition.
With the partition the computer becomes more stable, it may crash once a month at most, just because you are respecting the location of each file, something that fragmentation would damage after certain time.
The worst thing is to have everything in the same partition, even software and files. The manipulation would lead to some kind of disaster later on.
I hope this have been helpfull to you.
In your case I would add a 1.5G of space (at list) to OSX. You have to calculate some room for future fonts and extentions.