Yes, as you're running the v6 of the router, you'll need to use the vxworks_prep_gs_v03 before you flash it with the dd-wrt firmware. It's not complicated, and doesn't take long to do at all.
There are step-by-step instructions on what you need to do here...
http://www.bitsum.com/openwiking/owb...?WRT54G5%5FCFE (scroll down to the section "For the WRT54GS v5, v5.1 and v6 ONLY").
However, it's worth noting that for the GS routers, this process can
not be undone, so there's no way of going back to the Linksys firmware if you change your mind.