I figure I might as well not waste space on quality that doesn't exist.
If they were already compressed lossy before, then as high as possible to minimize further throwing away data. There's really not much else you can do here I guess.
While I agree, as high as possible would be ~500 kbps, which I think would be overdoing it a little since Mp3's maximum bitrate is 320kbps, and Ogg seems to have higher quality per bitrate to begin with. Should I just go ahead and do 320kbps Ogg, or would lower like 256 be suitable? If no one is really sure, then I will just do a few listening tests I guess.
I would think it heavily depends on the file, but yeah you'll probably be fine with 256 OGG files (especially if the source was indeed not necessarily 320 CBR MP3).
If you want maximal quality, you should rip to a lossless format.Anyway lossy format - regardless of compression level - will induce additional loss.