My two cents:
The most important thing is that you use version control. I have been a happy Subversion user for three years now, but eventually found myself frustrated with its merging facilities and am now learning Git (since it allows you to cherry pick the changes you want to merge).
For your purposes, however, I would not recommend Git (not quite cross platform since it lacks a Windows implementation, I think) and its UI is not as nice as some alternatives.
I think you might try Mercurial or Bazaar.
There are advantages to using a distributed version control system as opposed to a centralized one like Subversion, but really any of Subversion, Mercurial, or Bazaar would suit your needs.
They are all easy to install. Since you don't have Leopard yet (which has Subversion preinstalled) you can use Martin Ott's OS X binary.[1] The first two chapters of the Subversion book (available free online) should be enough to get you working with subversion in an afternoon.[2]
There are binaries for OS X and Windows for Mercurial (I think that Mercurial also comes with keychain support on OS X).[3] Like Subversion, it comes with a nice manual.[4]
Finally, Bazaar also comes with binaries for OS X and Windows.[5] And also has extensive documentation.[6]
You might be well served by downloading them all, spend a weekend playing with a couple toy repositories and think about how they might work best with your envisioned work flow. (Bazaar seems particularly flexible in this regard.)
Glad to hear that you are seriously considering version control. Good luck.
All the best, Mark
[1] http://homepage.mac.com/martinott/ [2] http://svnbook.red-bean.com/ [3] http://mercurial.berkwood.com/ [4] http://hgbook.red-bean.com/ [5] http://bazaar-vcs.org/Download [6] http://bazaar-vcs.org/Documentation