> I can get one of these free and
> swap my version 4 for a version 6.
Why the swap? See the wikipedia page for hardware version info:
http://en.wikipedia.org/wiki/WRT54G
If you're not using it for anything other than what the stock firmware
provides then it might not matter. But if you wanted to use a 3rd party
firmware for added features then it's best to stick with version 4 or
earlier units.