I've worked in a lot of untreated rooms, and some really bad acoustic spaces in my time as a music producer. Rooms with a lot of hard surfaces in particular are problematic, as are very large rooms. If you have the option to, you should go for a medium sized room and setup so that your speakers are firing the long way down the room. Invest in some broadband absorption foam panels, and place these on your side walls and behind your speakers. Place some diffuser panels behind the listening position, on the far wall, and some bass traps in the corners if you can. Finally, make sure that your speakers aren't too close to that wall, and that you have an equilateral triangle between the listening position and the two speakers.
If you follow these general guidelines (note that all rooms are different and require that you take this into account) you'll find that your mixdowns will improve a lot more than if you had simply upgraded your monitors and placed them in an untreated room. No matter how good your monitors are, they're only as good as the room that you put them in. With a treated room you'll find that you don't need to work so hard to get things like your sub bass to sit right in the mix. You won't need to take your mix out to the car or other listening locations as much as you have had to do so previously, and most importantly when you're mixing you won't be compensating for your room.