the title says it all:
how does mp3pro perform versus OGG RC3 at lower
bitrates?let's say, at 64kbps?

I can't tell any difference with my cheap speakers...

my experience has ogg sounding a fair bit better than mp3pro down around 64 kbps. To test it yourself, encode at -q 0.0, which is nominally equivalent to 64 on ogg's quality scale, except that true vbr is implemented. mp3Pro does better than wma (which Microsoft claims is cd-quality at 64 but actually is quite annoying and artifacty), but mp3Pro has a few more artifacts than ogg and has noticeable fake brightness. Monty expressed some disappointment with ogg's current performance around 64, but I hope that he doesn't go vey far in exchanging the current fidelity for an artificial brightness. But ogg will still be better than mp3Pro with that change (and I'll probably end up liking it more than the current ogg, despite my misgivings).