Will the newest one work or will I need to download an older version?
newest should work just fine
When selecting a board type in the IDE what you are looking for matching is the processor chip type and the speed it is running at.
That should work with whatever brand/model of Arduino compatable board you may be trying to use, or even a DIY standalone/breadboard version.
Didn't some of the later manufactured Decemilia boards come with a 328 chip as an upgrade from the 168 chip as standard? I'm not sure, but I think that is the case.
The early Duemilanoves had 168s, I can't see them overlapping the chips on both boards. Its not an issue, its the chip, not the board that matters, you just lie to it and tell it you're running a Duemilanove with a 328, if you happen to have a Diecimila with a 328 (bought an upgrade chip ?). It works well enough for my homebrew boards with 328s on them. :)
you just lie to it and tell it
I agree and is what I was trying to convey in my first response. Match the chip and speed and you will get the correct core libraries and AVRDUDE will see the proper processor chip type response when it tries to upload.