It's officially confirmed (and as Robpop stated earlier): Aqua's new greatest hits (out June 15) will have three new songs on it, including "Back To The 80's," which is the lead single and comes out May 25, as well as sixteen remastered older songs. The group performed the song at a comeback concert which took place almost a year ago.
To be honest, it's fine but I don't madly love it; still, I'll have to hear the production details to know what it's really like.
If you live outside Scandinavia (and in one of the countries on the list on this page, which includes the U.S.), want their greatest hits, and are willing to pay high import prices, you can pre-order it here.