You get one shot to reset your reputation with the Legion, and one shot to reset your reputation with the NCR. Once those are gone, everything else you do will be remembered.
If you intend to do multiple quest lines off a single save, do not do things that vilify you if you are already past those two points.
There are quite a lot of things you can do to gain XP and even gain standing with the factions without losing reputation with other factions.
Thankyou for the reply, much appreciated. If you can remember is there any chance you could name the point please, without spoling too much? Perhaps the quest name, if possible. I have the Platinum Chip, and have a quest from Wild Card (Independent) The House Always Wins (Mr. House) and the Legion. I have to go somewhere for the NCR questline. But I havn't bothered, due to reasons I stated before. I honestly don't think I've advanced far enough into the story yet though. Hopefully.
Oh, I'm liked by almost all minor factions and towns with exception to Powder Gangers, and I have no standing as of yet with the Legion. But the quests I need to do will change this to a negative stance. Will I be able to revert to neutral once I undertake that questline?