Hey Guyz ,
After Long Searchin And Researching I found a satisactory Converter For our N900 Videos For smooth playback of MKV in 600mhz.

Yes I cant Say its useful for everyone but try it out :

MKV Converter Studio

Its a good app which gives satisfactory results in less size [ ofc MKV]

Needed Apps On Device :
Extra Decoders Support
Any Video Player Of Ur Choice.(VLC Media Player Is Recommended if Soft Subbed Using MKVmerge)
Subtitles Support (If Needed)

So Lets Start Converting

First U Need:
MKV Converter Studio
MKVmerge for muxing subs
It Has Own Subtitle and audio track selection

About Profile:
Choose Any Profile
mp4 or mp4/avc or mkv
then choose bitrate of 275-500 (Avg Size) Or as u wish
Then audio i say 48 or 92 kbps is fine
framerate to 25
Resolution 320x240 or 480x320 is fine
and other options etc
and there u go U have a good quality Video output ..

How do you install Extra Decoders Support?

I installed extra decoders from the application manager but it wouldn't read my flv files. So I removed it.

Better try With Some Other Player Like KM player or VLC coz mediaplayer needs overclocked profile to play those files(if Showing media error)

Perhaps for those who are slightly less noob, my own favorite is Handbrake. Handbrake is really quite powerful, but it doesn't require too many steps to coax it into converting a video into exactly what your N900 desires. In fact, using this, there's no need (or point) to loading extra decoders. The major steps are these:

1) Open your source video in Handbrake.
2) Select "H.264" as your video codec.
3) Under the "Advanced" tab,
3.1) set "Maximum B-frames" to 0, and
3.2) turn off "CABAC Entropy Coding", "8x8 Transform", and "Weighted P-Frames".

You are now set to convert the video into an H.264 "baseline profile".

For the rest, I normally do the following:

4) For high-definition video, I only reduce the size enough to just fit it into the 800x480 resolution of the phone. (800x448 is the perfect choice for modern 16:9 widescreen video, to leave the aspect ratio intact.)
5) Choose either MP4 or MKV for your container format, both work fine.
6) I leave the frame rate at the same rate as the source video, this usually works for me.
7) Not being an audiophile myself, I usually let Handbrake do its default effort on the audio track...

The only real problem I have is with subtitles. As Media Player doesn't handle them (at least not without serious help), I normally just let Handbrake "burn" them into the video itself.

Media Player on the N900 has no trouble playing H.264 baseline profile video at 800x480 resolution, with decent frame rates, with decent audio, no need to load any extra packages, no need to overclock, no need to do anything. It just works.

to OP,

not to sound mean but seems you are stll in the n73 era...with stock clocks the phone can handle AVC baseline profile up to 5 mbps @ 800xXXX resolution, standerd mp4p2 can go even higher.

also note theres a big difference in plain mp4 and avc encodings at the same bitrate, do some reading.

mp4 container is still very relavant, device may have trouble reading new bits of mkv container. hence mp4 is suggested.

Forgive my ignorance however if you are going to go to the trouble of tanscoding a video why not just transcode it into something the n900 can play well using hardware acceleration?

What you are proposing is reducing the quality so it can be decoded in software?

Why would you do this?

Please all, before posting, read here:

http://wiki.maemo.org/Manual_video_encoding

and

http://wiki.maemo.org/Video_encoding

@OP - this isn't OSS, very windows orientated and most importantly, doesn't do a very good job! Mmm, perhaps we should agree to disagree?

Off-device, Handbrake sounds like the best solution. There are m$, mac and GNU/Linux binaries available.

As far as playing media, the application manager doesn't open flv files, the media player does. These files are in fact just containers, usually for mp4 or mkv files, which are themselves containers binding the audio and video parts of a file like a film.

There are a few things to note, including ...
- The super-container e.g. flv (for flash media).
- The file type e.g. mp4, mkv, avi, etc.
- The underlying media encoding e.g. h.264.

The stock player will play flv files, so long as the content is playable, which usually means it has to be mp4/h.264 baseline.
Files with the mkv extension do play in the media player too, so long as their encoding is appropriate.

I haven't been able to find a way to display the profile of a video on-device but try with WinAmp > View file info. The easiest way to find out is: does the video fail to play in stock player, yet succeed in KMPlayer? If so, it's the profile!

There are some useful commands that are easy and won't break anything...
- See which encoding the media is prepared with (audio & video):
mplayer -identify -frames 0 movie.mp4 - requires mplayer
- Extract content from flv container (transcode)
ffmpeg -i <filename>.flv -acodec copy -vcodec copy <filename>.<ext> - where "ext" is the encoding type detailed by mplayer's identify. Very useful for audio files because if you lock the screen flv files stop playing.

I posted this question in another thread but it was a bit off topic there. Is there a way to convert files on device? The following command doesn't work, where video01 is 500kbps mp4/h264:
ffmpeg -i video01.mp4 -acodec copy -vcodec copy -vpre "/usr/share/ffmpeg/h264-baseline" video02.mp4
- do I need to use some extra commands? The input and output are both mp4/h.264.

flv files play on my windows machine not on my N900. Currently I use Firefoxe's "download helper" extension to convert my flv files to mp4 or mov. The mp4 quality is good. The mov quality is better although the file size is larger, but I have plenty of room on my N900. The conversion process runs from windows to DOS so it's relatively fast.

Copernicus's settings on Handbrake produces a quality mp4. But since the conversion process runs on Windows, it takes a long time.

Would running the linux version of Handbrake be faster? Since I'm windows I'm not sure how to make a linux boot cd and get handbrake on there too.

The only difference I could imagine is whether or not it takes advantage of 64-bit instructions on Windows (I know it does on Linux and OS X). If you have a 64 bit processor (and most modern machines do), you might gain some benefit by running Linux.

BTW, I should also point out that while setting up a baseline profile H.264 codec in Handbrake will produce a quality video, it won't necessarily produce a perfect video. There are many, many options available in Handbrake you can use to fine-tune your converted video towards perfection. As gregoranderson pointed out, there are documents available which go into greater detail about this process.