I used that exact motherboard (A7N8X Deluxe) in my HTPC before I upgraded late last year. The sound quality is very good, and you have the Dolby Digital encoding feature already. I wish I could have used SoundStorm in my current setup, but I upgraded to an Intel box. I had an extra Audigy 2 lying around, so that's where it went. It was not my first choice, but it does the job.
I have the Creative X-Fi Platinum sound card in my main PC, and I can honestly say the quality is greatly improved over the Audigy 2. Worth every penny. If you game with headphones at all, I would strongly suggest buying the X-Fi immediately! I have never heard better 3D positioning through headphones than with this card. I used to game exclusively with a 5.1 Klipsch ProMedia system, using expensive Sennheiser HD-595 headphones only for music. I hated losing the 5.1 experience when gaming. Once I got this card things have changed, and I now game almost exclusively with headphones! Keeps the wife happier and the volume where I like it ..
I feel more immersed in the game with headphones and the X-Fi card than I do with a dedicated 5.1 setup.
Sorry if you are still confused.
Only you can decide what fits your situation the best. If money is an issue, I would use the nForce2 sound for now and get the X-Fi with your next upgrade. By then it will be cheaper and you can still have great sound now.
You can't go wrong with the X-Fi either. It doesn't have Dolby Digital Live, but if you are running stereo it won't matter.
BTW, you have already made the best decision with the 170SE's .. they are great speakers!