Are you using Tiger or Leopard?
If Leopard then it's easy bit a little time consuming. What's on your other partitions? If they're empty or have stuff that you can just copy over to your main partition then you can just select and delete them in the little partition map picture in disk utility. However, that can leave "free space" floating around that's difficult to add to another partition, though you can expand your other partition/s into the free space I found that about a GB remained free.
The "cleaner" way of doing it - cleaner in that it doesn't leave bits of free space floating around - is to back up everything (with Time Machine for preference) and format your harddrive then restore with your Leopard boot disk from the TM backup.
Of course, that doesn't work if you're using Tiger. I suspect you may have to backup the long way, format and reinstall everything the hard way. On the other hand, I haven't used Tiger much, so there may be a quicker way I don't know about.
