Thanks rlf, I got there just ahead of you.
Although I did it another way on my Dell Optiplex GX620...........but with the same end result. The internet WORKS!!!!
1) Downloaded the aforementioned AppleBCM5751.kext on my Windows PC (could have also used the working OS X on my 80GB HDD)
2) Didn't unzip it there because Avira Antivirus told me there were some bad things in it like worms (false positive?)
3) Saved the Zip file to a USB stick
4) On my Dell, copied the Zip file to the Desktop and unzipped it.
5) Looked in Systems/Lib/Extensions to see if there were any previous 5751.kexts or 5751.bak.kexts. If so, you should put them in the trash. Not sure if it's necessary to run Kext Utility after deletion. (comment, anybody?)
6) Shut down the Dell, rebooted but immediately went into BIOS F2 (System Utility). My BIOS version is A11 - might be a good idea to upgrade yours to the latest from the Dell website. Only downside is you need to be running Windows to facilitate the BIOS update. AFAIK, it don't work in OS X.
7) Disable the Network card (NIC), save those BIOS settings, and reboot. Hover your mouse over the 5751.kext (not the Zip file), right click and choose Kext Utility. Enter your password and have a cup of tea while KU does its stuff.
8) Shut down again, go into BIOS re-enable the NIC card to On (don't choose ANY of the other settings for Network boot), save BIOS settings.
9) Restart MAC OS X and your internet should work. Double check in SysPrefs>>Network and also Network Utility and of course by opening a browser.
Don't know why this procedure works (why the hell would an OS have to have the PCI card disabled before it can install its driver?? Weird!!!)..but I don't care.
The only fly in the ointment that there is no Mac address assigned in the Network Utility window in the field "Hardware Address" - it's all zeroes. We shall see if this causes any probs in the future. But the IP addess is automatically and correctly assigned.
Wow!!! 10 days of tearing my hair out is now over.Go to the full post