According to DD-WRT wiki, it's WRT54G version 7 and above that has VXWorks with 2MB of ram and the atheros chipset as opposed to the broadcom chipset in their earlier routers:
http://www.dd-wrt.com/wiki/index.php...RT54G/GL/GS/GX
From this wikipedia article it looks like the WRT54GS went to VXWorks with v5 instead of v7 which may account for the confusion:
http://en.wikipedia.org/wiki/WRT54G
At any rate, be sure to check out your serial number against this list before you try to flash anything:
http://www.dd-wrt.com/wiki/index.php...RT54G/GL/GS/GX
I would trust the DD-WRT info over wikipedia as these are the guys that actually hack the firmware upgrades.