Before getting a Mac, I'd take Andy Stitzer's excellent advice and give Ubuntu a try.
I was in the same position as you. Had doubts about Vista and thought of buying a Mac. When I looked at what a Mac costs...
... I decided to give Ubuntu a try, since it's FREE and it was the best decision I ever made.
jwmorrice posted a good article of how to tweak Vista to get peak performance. While I could do all suggested, others may feel intimidated to give those suggestions a try. I have Vista and really like it, just seldom have to use Vista preferring to use Ubuntu on my laptop's dual boot system.
With Ubuntu you have none of that Vista tweaking to do. You just turn on Ubuntu and run it, it just works.
Dual boot is the way to go. Just create a 15-20GB partition for Ubuntu and you have two operating systems. This way you can learn linux at your own pace with windows as a backup. At this point I don't need MS or Mac at all and linux is all FREE and does everything I want.
For the last month I've been using another linux distro openSUSE11 which I like but am getting disenchanted with. SUSE requires too much tuning up and tweaking compared to Ubuntu. Maybe it's because I've only used SUSE about a month compared to using Ubuntu for over a year. Things just run better on Ubuntu compared to SUSE but I wanted to give SUSE a try since it's the next most popular distro after Ubuntu.