It all depends on how you want it -- some people are going to tell you not to use partitions, and some are going to tell you to use partitions, both for different reasons.
- you can move data to a partition other than your boot partition... if your OS dies, your data is intact on a different partition. If your hard drive dies, you lose all partitions
- you can organize more effectively, much like having multiple disks... one partition for the OS, one for data, one for client's projects, etc.
- you can make a small, emergency boot partition that you can boot from in case of OS errors to run a thorough check on the affected partition
- multiple partitions do not provide any kind of speed increase by moving swap files, home folders, etc., contrary to belief
- it's complicated
- you can run into "gotchas" that require wiping the whole drive anyway, like making your boot partition too small then running out of room later
- it's better to leave OS X pretty much alone and not move certain folders from their default locations... you can back up your home folder on a DVD most times, and moving folders around sometimes confuses OS X
Anyway, these are just things I've heard. If you do decide to partition, make sure you make your boot drive big enough to grow as needed... if you intend on installing iDVD or GarageBand and leaving their gigabytes of support files in the default install locations (on the boot drive), that can take up 6 to 8 GB of space... I would recommend at least 30 and preferrably 40 GB.