Versions
There have been many versions of the WRT54G, as can be seen from the following link...
http://www.dd-wrt.com/wiki/index.php/Supported_Devices#Linksys_.28all_the_rest_that_is_not_re-engineered_til_today.29
Yes, newer version don't run Linux, but that doesn't mean one can't re-flash them with custom Linux-based firmware like DD-WRT, Tomato, etc., although, as can be seen from the link, the much needed flash (and the less but still useful RAM) size has been reduced in newer versions, limiting their usefulness.
To overcome this, Linksys came out with WRT54GL (note the "L" meaning "Linux") which is basically version 4 with a slightly different CPU.
Call me whatever you want, but somehow I doubt that the new WRT54G2 will provide more flash (or RAM for that matter) than the earlier models.