I have the Auzentech card in my HTPC and it kicks ass. It will definately beat your onboard soundcard when it comes to gaming-no other card on the market takes 3d audio and converts it to DTS-not even the most expensive soundblaster.
There is no such thing as future proofing your PC. If you are spending more than you want to, you can save money on the processor and video card-then next year you can upgrade and probably end up spending less money total. Just do a little research on the components and see what their price has done over time.
I don't see a power supply. Get a beefy, yet quiet one and a good case. Save money on your processor/video card for this if you don't have the budget. Like I said those two components are notorious for dropping in price quickly. 600 watt quiet power supplies tend to keep their value. Kinda like Speakers vs AV Receivers-the receivers lose their value quickly.
