I'd say use a voltage multiplier to boost the compter PSU 12V rail to 48V or 24V for the second one. Really, a computer PSU is a rather precise regulated power supply, don't waste it. Also, do attach a small heatsink, because the devices will probably have higher distortion and lower SNR if the temp goes too high, even if the devices are rated to handle the temperatures. I have routed things in my computer, and made a PWM fan controller before, but that's rather off topic...also, most high-end audio cards do have a EMI shield. It is not hard, a simple metal shield, ie. faraday's cage should suffice. Also, for home theater setups, you might want to go with a horizontal case. I suggest a SINGLE RAIL power supply, because a high wattage triple rail or quad might not have enough juice on one rail, and the CPU will be wasting a lot of juice, by dedicating one rail to itself.