How do I partition an external HD?

I would advocate getting two external hard drives both big enough to handle all your data, and not partitioning. Because it has been found that sometimes one drive may fail before you successfully migrate the data to a new machine.
Also be careful to observe the license agreement of any software you include with your old machine. When selling your old machine, include all the original disks which came with it. The buyer may need them at some point in the future to repair any problems which might happen in the future.

When you get your new machine, it has a migration assistant that can pull information off a Firewire hard drive. Though it is better if it is all on one partition when it does so. Partitioning doesn't really make it any safer. Partitioning should only be used when you want to create a scrap space to write data to that can easily be rewritten or have two operating systems installed that you can easily toggle between if you find some older software doesn't work with the newer operating system. That said, each Mac has a limitation of how old an operating system it can run:

http://docs.info.apple.com/article.html?artnum=25517

So it won't be exactly the same as you have now, but can have all your old documents and applications you don't end up selling with the old computer.

It's a reasonable plan if it fits your particular needs. You can partition the drive using Disk Utility. The procdure that I recommend is as follows:

1. Open Disk Utility in the Utilities folder of your Applications folder.

2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.

3. Set the number of partitions from the dropdown menu (use 1 partition unless you wish to make more.) Set the format type to Mac OS Extended (Journaled, if supported.) Click on the Partition button and wait until the volume(s) mount on the Desktop.

4. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list. Click on the Erase tab in the DU main window.

5. Set the format type to Mac OS Extended (Journaled, if supported.) Click on the Options button, check the button for Zero Data and click on OK to return to the Erase window.

6. Click on the Erase button. The format process will take 30 minutes to an hour or more depending upon the drive size.

Repeat Steps 4-6 for each partition you create.

You can clone your hard drive using the Restore option of Disk Utility as follows:

1. Open Disk Utility from the Utilities folder unless it's already open.
2. Select the startup volume from the left side list.
3. Click on the Restore tab in the DU main window.
4. Drag the startup volume to the Source entry field.
5. Select the backup volume from the leftside list and drag it to the Destination entry field.
6. Check the box to Erase Destination. Skip this step if you've already formatted the drive or if you cannot format it.
7. Double-check you got it right, then click on the Restore button.