Static noise in headphones

I have a Asus P6T based system. I was using the on-board sound (plugging in Logitech X-230 2.1 analog speakers in the green "front speakers" 3.5mm analog output, then plugging in my headphones in that). I was quite happy with the sound quality (didn't hear any static noise if volume was turned down to my normal listening level).

Then about a week ago I started having terrible static noise from the left channel, and no normal audio on that left channel. Right channel had more static noise than usual but did have a bit of sound. I tried using the AC'97 in front of my case but that seemed to have no signal.

I decided my on-board sound card has gone bad and bought an internal sound card to replace it (Startech 7.1Ch PCI). This fixed the "no sound from left channel problem", but I had much more audible static noise.

I decided the card was low quality and/or it had interference from all the other things happening inside the computer case (it's crowded), and bought a Sweex SC016 external USB sound card.

But even with that I have static noise in headphones. Positioning the USB sound card differently doesn't seem to help. Trying the other analog outputs (e.g., surround) doesn't help.

The static noise in all cases is proportional to the volume.

I have tried different headphones, but the situation is similar though perhaps the flavour of the static noise changes slightly.

So what are my options?

a) Get another, more expensive, external USB sound card hoping the quality will improve?

b) Get another, more expensive, internal sound card (PCIe 1x perhaps) hoping the quality will improve?

c) Get a dedicated DAC box?

d) Get some Hi-Fi earphones?

Suggestions?

tl;dr - Two different sound cards both still have static noise in headphones.