Want to cool my case need fan input

Hello there folks,
my PC started "wailing" a few weeks ago and I suspect it is all the dust and stuff. I run with the case side open (no idea why) so there is a lot of dust inside the poor thing.

Anyway I wanted to ventilate poor ******* a bit and also close the side panel as I can't stand fans anymore. I know adding fans will add to sound level but I hope by closing the side panel it will dampen the sound a bit and by keeping it ventilated that I'll lower the decibels

I have a lot going on in my case

Opteron 165 @ 2.55
2 Gig RAM
8800 GT 512MB @ stock
3 harddrives,
floppy
DVD-Rom
CD-ROM

So a lot of cables and stuff inside.

Anyway I think I have place for at least one fan at front and 2 fans back ( it may even be 2 fans at front )

and these are the fans I can choose from:

https://www.inet.se/datorkomponenter/kylning---moddning/chassieflaektar/

So what should I do? Go for 4 fans or start with 2? More intake fans or more blowing out? Any tips are greatly, greatly appreciated
Ofc before I add these fans I'll clear all the dust out first

Thank you for your time!

Noctua NF-P12 1300 Silent Case Fan 120mm





Scythe 120mm Ultra Kaze Fan, 3000rpm


These are good fans and fairly quiet too

Ok a little update... I can only mount 3 fans and I can only mount 80mm fans

I can have one at the front and 2 at the back

So what fans should I choose and how to setup them, 1 blowing in and 2 out right?

Or should I just leave it all and run with the case open?