I would get the Xonar DS over the X-Fi Xtreme Audio. The XA is repackaged Audigy SE. It uses a cheaper chip than found in the original Audigy, which is about 9 years old now. I used to own an Audigy SE, which I got rid of due to it's gimped performance in Win7. I replaced it with a Xonar DS, which soundly beat the Audigy SE in the sound quality dept.
The DS is a good card for someone on a budget, but it is best used with speakers. You can use a headset/headphones fine with it, but it doesn't have Dolby Headphone, which is found on the DX/D1. Dolby Headphone does what 5.1 phones do, basically. 5.1 phones use multiple drivers to give the user sound direction, but this can be achieved using just a pair of stereo phones with DH. Creative X-Fi's have a similar thing, CMSS 3D, bar the Xtreme Audio. In short, the surround sound virtualisation, as it is known (Dolby Headphone & CMSS 3D), fools the brain into thinking the sound is coming from a specific location.
5.1 headphones work to a point, but it's when someone decides to use them for music that they fall flat on their proverbial face. If you take a £50 pair of stereo headphones and a £50 pair of so called 5.1 headphones, the 5.1 phones will have multiple drivers, whereas the stereo ones will have 2. Common sense would say the quality of the 2 drivers found in the stereo phones will be much better than a load of cheaper drivers found in 5.1 phones. With a sound card that has surround sound virtualisation and a pair of nice stereo phones, you can have directional sound placement in games, but also phones that will serve you well for music. You don't have to spend £50 thought to get decent stereo phones, Goldring DR50's cost £20 and provide good sound for their minuscule pricetag. They are over ear too, which I find is much better than on ear when using Dolby Headphone/CMSS 3D.
You can use a headset with DH/CMSS 3D, but one problem is that headsets are designed for chat and gaming primarily, meaning music capabilities are left by the wayside somewhat. There are not many headsets that will perform as good as a decent pair of headphones, the ones that do are £60-£70 and more.
Just some ideas to think about.

If any of that make no sense what so ever, I'll try my best to explain it better.
