i'm currently using some crappy earbuds that came with my tablet,
I have to use an adapter though, as they do not work properly on the N900.
They're loud. Sound quality, for that piece of crap that it is, is surprising, but not anything to praise about. Also, left side is louder than right side. Probably something to do with that mic. My friend has the same earbuds that came with his Galaxy Nexus (and he dislikes them) and I checked them to see if it had the same problem as mine, and yes, they all have the left side louder than the right.

Edit: I'm also using the original nokia "rubber things", as those fit better on my ear

The reason I use them is because they're way less muddy than the N900 original headphones, but it's just temporary.

I plan in getting AKG K450.
I tried them on my phone in a local store and I found them actually pretty good, but the price was 100€
Did some research online and it seems to be a good choice. They're around 50 euros on amazon.co.uk so I can't really resist.