Macs are good.... but expensive. If you want to go the cheap and free route, go either FreeBSD UNIX, Ubuntu Linux, or Gentoo Linux, or Linux Mint. Mint and Ubuntu are definitely the easier of the 4. Gentoo and FreeBSD can have the learning curve of a wall perpendicular to the floor at times =D. But if you stick to it, you will learn very valuable lessons on how OSes, applications, and computers work. Ubuntu and Linux tend to work at lower levels of computing with less abstraction between subsystems at times. This allows you incredible customization powers and far more than Windows will grant you.
In terms of security, you can't go wrong with any BSD kernels (Mac, Open, Free, Dragonfly, Net). Linux is a very solid and secure OS as well. Either one of those kernels are a significant security improvement over Windows.
As far as online security, create a made up fictional profile that you use for registration forms. Using proxy servers or Tor is also good for surfing. If you want security abstraction when using google, use this:
https://ssl.scroogle.org. Use SSL whenever possible. Use Firefox or Opera and their security plug-ins they offer. It doesn't hurt to have multiple e-mail accounts either with one designated for receiving junk mail.
Disposable e-mails are great services to use as well for one time registrations or orders.
http://www.sizlopedia.com/2007/05/27...mail-services/
See if your credit cards offer the service of virtual account numbers. This abstracts your real number, but allows you to make purchases online with virtual numbers linked to the real one.
I got TONS more tricks, but it all depends on how many layers of foil you want on your tinfoil hat and how shiny you want it to be =D.