Do not convert ogg files to mp3; in general, don't convert between 2 different lossy formats if you can help it. You would only need to keep wav files around long enough to organize a play list of how you want them burnt, so the larger size shouldn't be a problem.
There are probably simpler ogg to wav converters, but if you also need to crop some leading / trailing silence, I would suggest the freeware sound editor
Audacity.
If you are going to mix in any mp3s from Larian's site (
here and
here) or
Kirill Pokrovsky's website, you will need to normalize the mp3s to the same volume as the ogg files.
