'But i was told that i should check that the Ethernet card i will buy for the PC must be compatible with the built in one of the G5', you were told ridiculous information from an unknowledgeable individual.
'How do i know what does the G5 has?', you do not have to.
'What are the specs that will comply with the new ethernet card i will get for my pc?'. A scenario: You have two automobiles - a VW Jetta, and a Honda Civic (with no engine). Do you really need to know the dimensions, horsepower, weight, etc. of the Jetta's engine if you are installing an engine in the Civic?
Go to the store, purchase any 10 / 100 Base T ethernet PCI (if you have a desktop / tower) or PC (if you have a PC laptop) card and plug it into the PC (I am assuming the PC is running Windoze 98 Second Edition or later).
While at the store - purchase an ethernet router [sorry, I originally said 'switch (not a hub)' by mistake] and any additionally needed ethernet cables.
Assuming you installed the PCI or PC network card in the PC, connect an ethernet cable to the dsl modem's ethernet port and the other end to the router's WAN port. Now connect the G5 and the PC to any available LAN ports (of the router).
To allow the G5 internet access, consult
this link for step by step set up instructions. (At step 6 - click on the 'PPPoE Options...' button, review the default settings and make any desired changes).
To allow the PC internet access - consult any PC competent person (not the individual who spoke to you about the ethernet card, etc.) and / or PC related support web site.