Can anyone recommend an XP-compatible sound card with separate outputs for each channel? I will not be doing any recording; just sound mixing my own film soundtracks. I use Sony Vegas 6 and my receiver is a Kenwood VR-409 (to be used for driving the speakers). I am looking to spend around $100. My budget is tight, and I know I can't expect anything professional, but it will beat rendering AC3 files to pass through my Audigy.

any audigy with 5.1 computer speaker (the logitech one are great) will allow you to edit real time with monitoring of 5.1 sound.
you just need to install the ASIO drivers and use a program that support it.
ACID PRO 4 is very easy on this.
You get a square box with the loudspeaker icons at each corner, and you just need to drag the cursor in the box to have the sound going from one speaker to another.

How does it work, Giroud? I have the original Audigy (with only one stereo-mini connector), connected to a receiver through a coaxial cable. This is only good for stereo, unless I passthrough AC3 (which you can't do in real-time). Is it not possible to use my existing set of speakers?

Emre,
You need a sound card with 5.1 analogue outputs. Pretty much any card with these outputs should work - you just pick the features/quality/cost you need. You need to connect your reciever by analogue 5.1 connections.

As you pointed out, spdif is only good for 2 channel PCM unless it is compressed to AC3 (or DTS).

Once you have everything connected you'll need to setup the audio properties in vegas to send the right channels to the right speakers, etc.

Cheers.

I used to have a Soundblaster Audigy 2 ZS on my old XP computer and it was an awesome card - beautiful, crisp and clear sound.
I think I got it for around 120 CAD brand new, so it should be right within your budget and it has an analogue output (3.5mm) for each channel.

Are m-audio and Turtle Labs any good?

M-Audio and Turtle Beach are more or less at opposite ends of the spectrum. The lowest grade M-Audio tools are about the same as the top end Turtle Beach, which are about equal to an Audigy.
Audigy cards color the sound, and they're not at all accurate. They do sound good. Like Bose speakers. Great for consumers, useless for authoring.

m-audio it is... Are there any consumer solutions to getting real-time digital output from a sound card to a receiver? What about professional ones (out of curiosity)?

Some of the pro cards have optical output, but I don't know if they'll connect to a receiver or not. Never tried. Receivers are colored too, so useless for accurate sound authoring. Of course, if it's what you got, it's what you got, and what you got is better than having nothing. Audio snob that I am, my first solo album was recorded on a little Tascam 422 recorder, and it sold 600K copies. My first surround sound project was an industrial site that rebuilds Caterpillar motors, and that was authored on small plastic computer speakers. Listening to it 4 years later, it's terrible. But the client was happy. :-)

Anyway, the 410 (M-Audio) has optical out.