What do you say abt this Ripping I did...

Xvid 2 pass 995 Kbps

Ogg 5.1 (96 kbps per channel) it was indicating 420 kbps

What source do you have?
If it's a DVD you will have a 5.1 AC3 you can copy without recompression.

420kbps for 5.1 Audio? How many kbps does the 5.1 AC3 have?
Usually something about ~384kbps up to 448kbps ... no recompression needed.

If you use Matroska it will be NO problem to multiplex AC3 via Mkvmerge(GUI).

995kpbs for video? (i hope you did resizing)
Maybe it's better to use BeSweet for downmix audio to 2.0 and compress it with Vorbis, so you have more space left for video.

I think it's a serious overkill.
First, you should rise up the video bitrate a little. Audio bitrate is, from my experience, 56-64 kbps per channel enough, especially if you use Vorbis.

if you want to use 5.1 leave the ac3 as it is.
vorbis does not have any special channel coding for channels > 2

If u want to convert an AC3 5.1 448 kbps stream to Ogg Vorbis 5.1 Stream ...

Wha tis the optimum bitrate to choose?

that was allready answered, there is no optimum bitrate, the answer is: dont do it, leave ac3 as it is.

Wait! Hold on! If u mux an AC3 it'll be around 400 - 600 MB depending upon the bitrate and the length, but I cant give that much ratio for Audio, instead vorbis 6 ch 280 kbps saves 200 mb approx. What I actually want is More Space for VIDEO and 6 CHANNELS AUDIO

I'd suggest aac then.

Ok u suggest AAC. BUT Y?

I mean how about the quality at low bitrates?

Like Vorbis is too good!

Recently I found that Vorbis is more than enough for me ... after encoding ac3 5.1 448 kbps .. to 224kbps 6 ch ... wid Awesome sound quality .. I dont think i have to switch to AAC

Vorbis doesn't have channel coupling (yet), thats why it's not really suited for 5.1. But if it's good enough for you, why worry?

Are there any developers working on that? I think that most contributions to Vorbis come from independant developers like aotuv, and that xiph.org developers have become quite inactive. But I may be wrong. And aotuv as the best- known developer works on enhancing the quality at low bitrates, not on channel coupling.

Aoyumi doesn't have the setup to be able to implement channel coupling for 5.1 files in Vorbis. He is only working on the low-bitrate tuning portion of the encoder, which is a xiph bounty. I tried digging through the source code to see what would be needed to get channel coupling working for multichannel Vorbis streams and I would even consider playing around with it myself, except it's very difficult in that it requires a lot of low level knowledge and additional submapping routines need to be implemented. The only person I could see who would bother to take the time and energy to do is the lead developer Monty and so far he has his hands tied right now. If you want to transcode from AC3 to 5.1 Ogg Vorbis you shouldn't have a problem if you use -q 6 and up, but I mean god for bid anyone read's the wiki . No channel coupling is applied so it shouldn't be a problem. If it were me personally I would Ogg Theora/Vorbis to do my video encoding, but Theora is still in the early stages too.