Like The Leander asked, I'd be interested to hear if you could ping that google IP from your Amiga.
Can you ping the DNS IPs you've tried from your Amiga?
Can you traceroute to the DNS IPs? What does that show?
I'm assuming the PCs pick up DNS settings properly from your ISPs DHCP, right?
And, what are the IP, Subnet Mask, and Default Gateway your Amiga gets? Are the Subnet Mask and Default Gateway set the same as the PC machines that work?