I did the stuff for the Kings last night, too, and it was annoying because I had to kill some NCR troopers. I'm working for the NCR, so that little stunt bought me some infamy
Oh, and if you do like 3 quests for the Van Graffs, you can get in on a part where you can slaughter all of them and loot the entire shop. Loooots of guns and armor to sell.