When you go to download Backtrack Linux you will find a variety of flavours and architectures to choose from. Backtrack Linux is a very well supported and constantly updated linux distro so finding support and help in forums is never to hard.
Before you download Backtrack decide if you are going to run it virtually in VM environment, or maybe GRUB boot it with your everyday OS. Personally the first time I installed backtrack i installed it on its own laptop, a Pentium 3 I had just sitting around and since then I only install it on its own independent laptop. That way I was forced to use it when i had it with and using it on a dialy basis helped me learn the ins-and-outs of BT. Also the first time I did some penetration testing with BackTrack i was able to just leave the laptop onsite without worrying about it being my main machine..
One issue I did run into was lack of support for some of my components on the laptop, some of which I could work through and other issues I had to live with. I bought a different wireless adapter as the built-in wifi on my old laptop didnt work well with BT. Just do some forum searching and googling and you can find out pretty fast what hardware will work best. Also I tend to stay away from the “newest” version of BT, for one very simple reason. The older version are a little better supported becuase people have been using them longer and for someone like me who isn’t an “advanced” user getting that support when I need it is VITAL
Anyways here are some download links:
BackTrack Download Wiki (probably the BEST place to start)
Backtrack Forum (everyone will eventually run into an issue and THIS is the place to find your answer!)