I would think the answer to that question would depend on your current abilities, and where you plan on your abilities being years down the road. I use a set of M-Audio BX5a moniters ($300 Range) and I'm no pro, but I think I my recording results are not hindered by my moniters by any means. So I have no means on upgrading at the moment, or anytime soon. The translation I get from different systems seems to be pretty accurate.

I think your question could be a little better answered if we had some more info like:
What do you currently use for moniters
What level of recordings are you trying to achieve
What is your budget?

Well i mix on a pair of Phillips SBC hn110 headphones, and a panasonic surround sound system. I also have a pair of tracking isolated sound headphones but the phillips sound better.
I want to be able to sell songs with them sounding good.
My budget is $400-500. I dont mind buying used gear from a higher price range.
Should i buy a better pair of headphones and a pair of monitors? or justfocus on 1?

Personally I would just focus on Moniters. But I've never even tried to mix in headphones either. Just always assumed it wouldn't translate very well? If I was you I would do as I did and go to a local music store...Sam Ash, Guitar Center. Anywhere that has a variety of different moniters set up to listen too. They you can pick what you like the best. There is a downfall to that also because most of the time these places are loud and are not going to represent what your studio environment will sound like.

You could spend a ton of money on good monitors, but if your room is not acoustically treated right they are never going to sound right. I would figure out your budget and set some money aside so you can treat your room when you decide on a set.