First, a theme written with 2.0 is going to be somewhat more forward compatible. My public themes were mainly written for WP 1.2.x and while I ported them to 1.5.x and they all work in 2.0, we're talking about some old code.
Much has changed in WordPress since 1.2 and writing a theme specific to 2.0 assures the user that the template tags, and pages will usually have up to date code, and be more forward compatible than a single page theme originally written for 1.2 or earlier.
Dig around in the codex and look for a changelog between 1.5.x and 2.0.x for the specifics.
If you are new to the world of themes, my suggestion is to have a good read through the codex for general ideas and thoughts about what can be done. book mark it so that you can refer back to it. think of the codex as the user manual.
Pick a theme you like, and have at it. destroy it. ruin the images, and mess up the code. Work your way back from there. In the end, not only will you most likely have a theme which is all your own (ok mostly,) but you'll have a fairly good working understanding of how themes work, and how to put them together.