@ Deepanddark20

I'm not a Lucas apologist in the least and I think the evidence out there quite clearly shows that they were making things up and changing things as they went along. I never said Lucas had some big overall storyline planned out from the beginning. He quite clearly didn't and has lied about it for decades.

But that's not the point and has little to no bearing on the specific topic of Darth being a name. It very well may have originally been intended to be a name, but there is no clear evidence within the OT, or even within the first film if you want to narrow it down that much to prove that point. The strongest evidence is Obiwan's line about "a young Jedi named Darth Vader, who was a pupil of mine..." But even that line makes perfect sense if you replace "Darth Vader" with "Captain Solo" or something similar.

JEDIT:
So I looked up the exact line and it reads, "A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi Knights."

That does NOT say what his name was prior to turning to evil. It can be inferred, but that inference may be incorrect. The only thing that can accurately be ascertained from that line is that he was known as Darth Vader when he hunted down Jedi for the Empire.