hi,
i am trying to play a .mp4 video file and have the extra decoders installed. however it doesnt play it. is it because the resolution is bigger than 800x480? i'm having trouble with some other formats such as .avi that dont play.

any advice/suggestions are more than welcome.

EDIT: The resolution of the video is 720x352.

Assuming by Media Player we're talking the built in Media Player...

What (if any) message do you get that it doesn't play? Format not supported or something else?

Also can you provide more information on how the file is actually encoded.

Additionally, if you end up going the route of converting the video on your desktop before transferring it to the N900, I've used the following on a Linux desktop with good results.
Mobile Media Converter
http://www.miksoft.net/mobileMediaConverter.htm

Handbrake
http://handbrake.fr/

Tablet Encode
http://maemo.org/downloads/product/PC/tablet-encode/

I know there are plenty of others and also for other OSs.

Yeah the N900 may not have enough processing power.

Don't waste your time with the built-in video player, install Mplayer/KMPlayer.

I used mencoder to convert videos (better do this on a desktop). This is a script I use to convert a 16:9 video with MP3 audio:

Of course if the video is 4:3 you might want to go with 320x240 resolution - it still looks pretty sharp. The compression artifacts are sometimes noticeable though, you could probably increase the bitrate quite a bit before it would bog down the CPU.

i should have provided more information. it says format not supported in the default media player. in the kmplayer it just doesnt play. i just bought n900 on the 5th so still learning

if i transfer the vid to my ipod touch it plays so there is something wrong here. also having trouble with few avi files as mentioned earlier.