Conversion advice for a newbie

[darafat]

first of all hello to everybody
Since i m pretty new to video conversion/editing i need a little help for what i would like to do, that s why i m asking here.
After years i got lots of video files wich have different codecs, and others that should be merged.
The problem is that i dont know how to set the best parameters for my videos, because anytime that i converthem into something i always make mistekes such as higher and useless size on the final video, wrong audio syncronization...
I would like to have for every video the same size without adding useless stuff, for example when i convert them on a higher quality. So what i ve understood is that before i convert them i need to put the some parameters such as the resolution, compression etc but i dont know how to get them and if they are enought.
Anyway the final result is that i would like all my video files in the same codec: xvid/mp3(lame) or divx7/mp3(lame) without losing quality.
How can i achieve that result? Do you think that configuration a good one?
For to convert i can use (i own) Avidemux, VirtualDUB, Nero, Sony Vegas.
I have another 3 problems:
a) I have some files in mkv so i dont really know how much quality i will loose if i convert them into xvid.
b) Some files are like corrupted and i m not able to convert them with avidemux or virtualdub because the program itself crashes
c) the flv video format with virtualdub doesnt load! i tryed the mod of a guy wich doesnt work aswell

I hope that some guru of you will help me! Now i have finally a new computer (cpu/gpu/ram) and i m now able to do this work, before i had a p4...........
Thank you very much guy! And any advice is appriciate

p.s: Avidemux is very easy and nice, but i ve seen that sony vegas does a good job but actually it looks complicated to me

[RabidDog]

that would be lose not loose. tern:
I dont think what you want is possible as whatever settings you decide on will either inflate the size of low qual video's or reduce the quality of your hi qual videos.
You can always go down in quality, but you cant add back detail/quality that has been lost. mp3 isn't really that good for keeping quality sound especially if it is more than two channels, AC3 is preferable.
I wouldn't bother and would just do any future projects to a high quality level.
Convert them (future) into the highest quality available for the playback device you have now. SD minimum??