I think getting a pair of studio monitors might be a good choice for you, as a sort of compromise. I know ATH m50's are often advertised as being studio, but they're actually a fairly long way off from having a flat frequency response. That's not to say they're bad headphones, just not the best for mixing, as was said before.
And as was also said, getting monitors in an untreated room is less than ideal.
So studio headphones will make mixing easier for you but also won't require too much setup, and aren't too expensive (you can get some models for less than m50's, and loads in the same price range). That's where my vote goes

(I personally use a cheaper version of Brainwavz HM5/Fischer Audio FA-002's. They're pretty nice and are really, really light and comfortable, so i don't have to worry about wearing the for hours on end)