Depending on the resolution of the video, your radio box should be able to handle it (assuming your internet connection is also up to the task).
An AGP card with S-Video output should be fine, but I'd keep it VERY basic and VERY simple to avoid any complications, look for a GeForce MX 4000 or GeForce MX 440 for AGP or PCI (prefer AGP of course), or a GeForce FX 5200, or Radeon 7000/7500 or 9200/9250, all should sport an S-Video output, and will be basic enough/low power enough to not conflict with the system (while a 7600GT should be compatable, realize that it'd probably use on its own as much power as the majority of the rest of the system).
For audio output, I'd take the best possible path, if that Soundblaster has co-ax S/PDIF out or optical S/PDIF out, go with that (if the reciever can take it as an input), otherwise I'd go with stereo output from the primary line out, get a 1/8" TRS to RCA stereo adapter (NOT a cable, most cables built to do this fall apart, the actual adapter device and normal RCA cables costs about the same and works far better) should be about $5 at radio shack, and any RCA cables you have on hand work fine.
You can try to mess with using more than 1 of the 1/8" TRS outputs, however the reciever has to accept 5.1 on RCA inputs, some don't (its a lot of connectors to have on the back of a device to do 5.1 on RCA).
So, I suggest stereo because it'll work for what you need, I get that it won't be surround sound, but it should work out just fine. If you wanted to purchase a different PCI adapter to provide 5.1 out over digital (if the reciever supports it), I'd look at Auzentech and Turtle Beach, ignore Creative, M-Audio would also be a good option, although the cheapest 5.1 card they offer is around $100 (while Turtle Beach has cards that cost around half that, and provide 5.1 (the difference being that Turtle Beach cards are designed for what you want to do, M-Audio cards are designed for the studio)).
One thing to realize though, is that your TV is NTSC, 720x480, which means from a computer's perspective its very low resolution, videos will look ok, however the Windows (or any other OS) desktop will feel somewhat constrained.
Here's a cheap FX 5200:
Wouldn't expect 3D performance miracles, although it will still surprise you with what it can play for $25.
And for a cheap digital out audio board:
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