That depends what you want to do with it. The original WRT54G (and WRT54GS) were based on a Linux firmware which was GPL'd and then had the source code released. The newer ones were rebased onto a newer platform to allow Linksys to reduce production costs.
If you want to use the third party firmwares then you'll need an older version of the router. If you are happy to run the Linksys firmware then there's no need to worry. If you have trouble finding an older version, check out E-Bay or look for the WRT54GL (the WRT54G with the original Linux based firmware - although Linksys now charge a slight premium for this) although I've just seen something saying they've discontinued it already
For more information on what some of the third part firmwares can do and the hardware versions have a look at LinkSysInfo (link above) and the DD-WRT WIKI (OK, I'm biased because I use that firmware
) at
http://wrt-wiki.bsr-clan.de/index.ph..._Docu_%28EN%29