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Seeing that the source is an Mp3 ranging from 128 to 192kbt does WAV offer any benefit over AC3?
Yes, the WAV files will be of better quality. The fact that the source is MP3@128-192kbps has no influence on that. AC3 is a lossy format and hence an additional loss is introduced. Actually, converting to WAV@48kHz isn't lossless either but the conversion to AC3 introduces a second loss as well. The process you are doing can be described this way:
1. MP3-to-WAV:
MP3(44.1khz*) [decode=lossless**] -> WAV(44.1kHz) [resample=lossy] -> WAV(48 kHz)
2. MP3-to-AC3 :
MP3(44.1khz*) [decode=lossless**] -> WAV(44.1kHz) [resample=lossy] -> WAV(48 kHz) [encode=lossy] -> AC3
* The MP3 is most likely sourced from CD and then 44.1kHz, but you know that better than me...
** Actually this isn't really lossless either, but this part has to be done anyway upon playback so it doesn't introduce any additional loss.
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I may switch to a service that has Ogg files.
It doesn't affect the answer I gave above regarding WAV/AC3. But the Ogg files
may be of better quality.
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Also, (I think I know the answer to this one) does upsampling an MP3 or Ogg to a different format provide any benefits for playback on my PC using Foobar?
No, you are only introducing additional loss.
If you are interested in storing your music on DVD:s with minimum loss you may be interested in
http://dvd-audio.sourceforge.net/Using DVD Audio you don't need to resample files@44.1kHz to 48 kHz as DVD audio supports 44.1 kHz as well. Only problem is that you need a DVD Audio capable DVD player.