I highly recommend that you do NOT buy a Mac.
I suggest a distro of GNU/Linux such as Ubuntu or Kubuntu. These are completely free, absolutely reliable, and have a major update every 6 months. If you are willing to learn a new OS, Ubuntu is better and cheaper than any Mac.
Download Ubuntu 9.04 (for free) and burn it to a CD. Put it into any computer and select "Try Ubuntu without making any changes into your computer." This will allow you to try Ubuntu without changing anything permanently. It will be a bit slower than it would be running from a hard disk drive.
You may think of GNU/Linux as requring lines of code in a console every time you want to install anything, but Ubuntu uses a packet manager where you can easily install programs that do everything you need, and update your OS every 6 months.
If you are worried about security, Macs are only less virus-prone because few write viruses for them, but they can be easily hacked into with a modem and rm / -rf. Macs are based on Unix, which is how they claim to be secure, but because they changed parts of Unix, they are not secure. Ubuntu is also free of viruses.
Ubuntustudio is what you will want to do audio, graphics, and video. Openoffice will replace Word, Powerpoint, Excel, etc.
I would only recommend Windows over Ubuntu Linux if you want to play games, or use Windows programs (although you can run those in Ubuntu through Wine).
For what you want to do, Ubuntu is superior (Kubuntu is also very good).