It sounds like the onboard sound is your bottleneck. I'd say pick up an X-Fi Xtreme Gamer card. If you buy a $100 set of headphones it's likely that it's still going to sound tinny due to the limited features of onboard sound. The onboard sound can only take up so much of the motherboard...
I agree with enginurd, the video card is overkill, and I'd scale it down to an X1950 Pro or a 7950GT, and spend at least $100 on a higher end power supply. The power supply is something you should NEVER cheap out on, trust me on this.
I own a 7600GT myself and it is a rock solid card, but it is admittedly not the card for someone who wants to run 1920X1200 at full settings and high anti-aliasing with every single game they can throw at it. However for HL2 and WOW you'll be very happy with it. I run HL2 on high settings and 2X...