So whats all the theory behind it all? I've tryed reading a few threads to calibrating a setup like this but everyone has a different opinion. My H140 iRiver (ownz the shizzle out of iPods!) has an optical out also, would this help at all on producing a good signal to the HU (through aux in)?
Are there any specific cable i should use going from MP3 Player -> HU or just a double male head phone jack?

Optical out will only work if you have compatible optical in on ur processor. this will cost a fair bit of $$. And for what its worth, the connections wont make an already compressed mp3 sound much better.

Try and hardwire it in preference to a fm modulator type setup.

What headunit you got?
Hopefully one with aux input via rca in the rear (or a cd changer input (pioneer, jvc, kenwood, Alpine Ai Net) that you can buy an adapter for to convert to rca)
Or via minijack in the front.

Im not too sure ont he iriver options for pluggin it into the car. You want a charger and preferably something that uses the line out over the headphone out. As the extra preamps for the headphone out will add to distortion.

Peace

Tim

I bourght a head unit with a minijack input on the front. So really i'm all set, but i just wanted to know whether there was anything i could do to stop any sort of quality loss between the mp3 player and head unit.

Not really dude, the irivers are great having under 0.05% THD or something I read so you should be safe. You will just have to go through the mini jack. And make a nice custom mount of course :D

hehe already done that. I should have everything up and running by next week, i'll post pics soon.