According to
http://www.bitsum.com/openwiking/owb...?WRT54G5%5FCFE
you should be able to flash DD-WRT onto your WRT54GS 5.1 (although there are warnings it is irreversible...).
There does appear to be some issues with some Linksys models on the stock firmware. I have DD-WRT on a WRT54G and regularly use a VPN connection with no problems.