Walt Mossburg has created a nice matrix showing upgrade options. This lets you see what your upgrade options are once you have decided what version of Windows you are going to buy. If you are a home user you can get the Home Premium version. If you need to connect to a domain, there is Professional. And there is Ultimate for those that need features not included in the first two editions. If you want to know what those differences are, I cannot help you.
The next decision is 32-bit or 64-bit. If your computer has a Core 2 Duo or similar processor then you can use either version. If you need to access more than 3GB of RAM then the decision gets easier because only the 64-bit version can do that. Shouldn’t everyone go with the 64-bit version? Sure, so long as you do not have old software that is incompatible with it. For me, the decision is paralyzing to the point that I have delayed buying Windows 7 in hopes that there will be more information available later before the beta version expires.
Apple released Mac OS X 10.6 Snow Leopard. There is only one desktop version. It installs and runs in 64-bit mode automatically based on your hardware and you do not need to have this decision. There is just one SKU that makes the $29 purchase a no-brainer.
I’m concerned about performance and compatibility of the 64-bit version knowing that this is the version I eventually want to be using. I need to use it in a dual-boot and virtual environment. One good news items is the fact that the product key for Windows 7 works for both 32-bit and 64-bit versions. So if you had the media, you could install either edition.