Nice. I'll chime in on the AC3/5.1 part.
Dayton spdif-toslink converter will run on 8v minimum. The mislabeled AC/DC converter puts out 9.3v.
If you use a front end such as Road Runner, try mapping the "master volume" slider to "wave" in the config if you want to control the volume via the front end, as the "wave" slider controls SPDIF. This is very useful if you only use software volume control, or if you want to use speed-attenuated control. Frodo also has a match volume app that can link the two sliders. Search for "MatchVol".
I disable Winamp volume control for one less slider to deal with. I also use Direct Sound output in Winamp as well as Media Player Classic. DS is especially nice if you use a Powermate or similar for fastforwarding, as the fade feature eliminates the choppy skipping sound.
Install WinampAC3. Mine is configured for output of Direct Sound, SPDIF, and PCM 24bit. Set it to 16bit if you don't play 24bit audio. I haven't found the "FLOAT" to be reliable. Don't use any of the Mixer or EQ features in WinampAC3 unless you need to. This way, you can send your music as unaltered as possible to your DSP. AC3 files played in this manner will be 100% untouched, wave volume and EQ settings won't change the output at all.
The Winamp EQ will still work with SPDIF output for all but AC3 files using these settings, which are the only audio files you can output completely untouched in Winamp (AFAIK, someone correct me if I'm wrong). I use the Winamp EQ for MP3s (and love my presets), but it shouldn't be necessary with an Alpine DSP or similar. The more done in hardware the better IMO.
Somewhat related: if you have DTS audio and want to play it in Winamp, install the "in_dts" plugin. DTS WAV files will bypass EQ functions, but are still controlled by the Windows wave slider, since they aren't routed through WinampAC3. This plugin downmixes to stereo though, so another player is the best choice. I use Media Player Classic or convert the files.
5.1 audio is absolutely amazing in a car environment, even without a center speaker. Your DSP can route the center channel sound to the front left and right, or you can do it in software with WinampAC3. I convert my SACDs and DVDAs to DTS WAVs, or simply grab the WAVs if DTS tracks are already available. They sound better than the AC3 tracks, but take up a lot of space. You can compress and tag them with FLAC and keep the six channels, which is a lot of work, or convert to AC3, which is the better option for Winamp. The Winamp in_flac plugin also doesn't seem to like 5.1. I haven't looked into the AAC format, but that might work with the appropriate plugin for Winamp.
AC3filter is not required if you have FFDShow (which uses the liba52 codec) and want to play video with mutichannel audio. But the two can both be installed without problems, which will give you the option of using each for specific audio types. If you don't have a DSP with advanced processing abilities, either can upmix stereo tracks to 4.1/5.1 as well, which can be very beneficial, for example sending adequate bass to your sub.