The more partitions, the better. That way you can put more OSs on the HD. Everone should have at least two instances of the OS he uses on the HD. Then he should also have two instances of the _next_ OS he intends to update to (so he can see what the pitfalls are going to be.) You need two of each OS (at least), so that when you screw up one, you still have the other working to help you fix the main one. (And everyone should be trying out linux on another partition, just to be familiar with it. A second version of that is also advisable, so you don't have to resort to a mini version from floppy when linux quits working. Although linux is not too useful as it is, it gets better every day, and it is the way of the future.) Also try out junk you are uncertain of -which should probably everything- on the secondary one. At the very least, you will at least know if the installer program works. New add-in cards should really be installed on a secondary COMPUTER before you commit to putting it on your work computer.
You should have one extra partition for putting a new OS on, just in case. That way you can do a clean, fresh install to see if that fixes a new hardware problem, without unnecessrily losing your treasured main OS install. Since in 95% of the cases, a clean install does not fix the problem, even though people always recommend it, you will be way ahead of the game.
If you work with video, no reason to put them on the same HD and risk your work if for some reason you have to reformat or change OSs, or think you have to. Get that giant HD and use it only for the videos. I mean, DO NOT repartition your HD to put linux on it while you have massive amounts of valuable data on the same HD.