Yahoo,
Just a small joke really. Personally I can hardly hear my HTPC over the LT150. Yes noise is additive, but only in a "I hear more noise" type. The loudness is I believe always determined by the noise source with the most dB and the distance to the source. In this case the LT150 (unless you're one of those insane overclockers that believe a CPU fan has to be rated at least 45dB before it can properly cool your processor) being the worst. The characteristic sound of the color wheel is also worse than what you'd get from a normal HTPC.
PCs can be relatively quiet by careful choice of quiet hard drives and fans. If necessary the fans can be speed modified. Both harddrives and fans can also be mounted in order to minimize vibration noise. You can also add sound dampening material to your case. I've heard several success stories of people using DynaMat for instance.
Really a stock pc is relatively quiet compared to the LT150 and I'd worry more about building a hushbox for that before you tackle the HTPC. If you're using floor mounting you might consider building a combined hushbox for the projector and the HTPC. I'll be modifiying an AV rack for this purpose.
Lou,
The scaling algorithm used by the software players on the HTPC is superior to the one used internally in the LT150 hardware.
As for audio. If you just use the digital out of your HTPC to pass the DD/DTS stream to the reciever they should arrive unmolested. There are some who claim that the ATI player delivers better DD out, but personally I'm one of those people that believe that digital is digital. Especially since AC3/DTS is just passed straight out and not modified in any way.
What you might notice a difference is for other sources such as CD audio and other regular audio applications where the audio is decoded/processed before being sent to the digital audio outs. If that's important to you check the HTPC forum for a recommended sound card with low noise etc. If you're going to be using the analog outs then that goes double.
jamin,
Sorry if I wasn't clear. What I really meant was that the internal scaling of the LT150 is not as good as the scaling of a HTPC. If you output 640x480 from the HTPC for instance you'd run into the same scaling artifacts as if you had a progressive player connected since the LT150 has to do the scaling internally to its native resolution of 1024x768. This scaling is independent of what connection you use - only the input resolution matters.
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/frode