Oké it is much clear now maybe I read your first post to fast because it was in there
You can indeed flash your router with DD-WRT take a look at this: http://www.dd-wrt.com/wiki/index.php/Linksys_WRT54GL
By the way, MAKE A 100% SURE YOU HAVE INDEED THE WRT54GL (if it is another one you get your router broken)
And don't forget to portforward the specific ports in your modem (of course when your vpn server is up and running)!!!
Another good solution can be to buy an NAS for the VPN .