Set your WAG120N into bridged mode. No NAT, no dhcp, no PPP authentication into your ISP, no nothing. Just bridged.
Then set your new DIR-645 as a normal router, which also performs the PPPoE authentication to your ISP. It will perform NAT and dhcp for the PCs in your LAN, so make sure you enable it to do so.
If that's not what you want to achieve and we are guessing incorrectly what you were thinking when you typed your message, then as burakkucat says post again with your requirements and we'll have another go.