If you're going to have more than 3GB or so of RAM, you'll want a 64-bit OS.
Even if you get eleventy billion thingies of RAM, you can still run 32-bit windows, it just won't actually use more memory than it can address. It does give you the option to postpone the 64-bit upgrade until you feel like it.
Windows 7 still has the 32- and 64-bit versions separate, and will likely have most of the same driver availability problems that vista does for awhile. However, it's free (to use) for a year or so, so you can try both and see if it's worth the fancy one yet. http://www.microsoft.com/Windows/Windows-7/download.aspx