320k MP3 is fine. If you said 128k, that wouldn't be good.
Thinking about it, the Essence is somewhat overkill if you are only going to use it with speakers. First and foremost it is a card for headphones. For someone with no sound card, then with or without headphones, it is a great card to have, but when you have a D2X already, the gains will not be so much I feel, decent hi-fi setup or not. I feel the gain in sound quality is there using speakers, but I don't think it's going to be a big enough increase to justify the cost. If you were intending to get some decent headphones at some point that will benefit from the Essence's amp, then that does change things somewhat.
If I were in your shoes, I think I would look to getting some decent speakers first. The Z5500 are good for what they are intended for, gaming and movies, but for music they are lacking. Either some monitor speakers like the AV40's or AE A2's, or an separate amp and speakers, and go from there. The D2X is still a very capable card, even when being used just for stereo.
The Z5500's deliver quite a lot of bass, too much probably. Monitors and smaller bookshelf speakers would be somewhat lacking in that area in comparison, is that something you would miss? Do like the ear destroying bass the Z5500's deliver? What monitors and bookshelf speaker lack in bass, they more than make up in mid range, which is where many 2.1/5.1 PC speakers fail. Should you want earth shattering bass, with certain amplifiers you can connect an active sub, so you get 2.1. I think Monitor Audio make a sub to connect to their monitors, but I am unsure if the A2's feature that connection. I think monitors like the AV40's or A2's provide probably enough bass for a lot of people, depends on the room size I guess. I don't want you to get the impression they or bookshelf speakers provide very little to no bass, it's just obviously significantly less than a dedicated sub can provide. Some people just like lots and lots of bass, which if that's the case, monitor or bookshelf speakers would disappoint.