So I started a new character, because with the previous one I experienced so many bugs (pre-patch) that I just wanted to start over. I finished some stages of the "tutorial quest" in Goodspring but I haven't helped Gringo nor Cobb yet, and I advanced further. I started to help out the Powder Gangers, building up my reputation: maybe I won't be vilified if I side with Goodspring during the Ghost Town Gunfight quest... Have anyone tried this to confirm?
I've tried it, you gain a few reputation points doing quests for the Powder Gangers, it didn't drop me down to vilified after doing Ghost Town Gunfight quest.
Spoiler It did drop me down to vilified after siding with NCR and taking over the prison. (when I killed a few of them) I'm thinking perhaps its not the best thing to do, but tell NCR not to attack. Since NCR will just be spreading itself thin everywhere trying to defend what they have, and will probably be steamrolled by the Legion when and if they attack. The Powder Gangers could act as a buffer, unless there is a way to unite the different factions, but I haven't figured out or played that far yet.