The rig will change hopefully because of your helpful answers to the Questions below which keep this rig uncompleted but each one solved helps.  
Orange needs attention.

THE PLAN NOW IS
It ll be mining only rig so no case etc but it ll have:
lots of noname PSUs prolly 1 per card so about 7 per rig.But what and how to check in a cheap psu so it wont kill itself along with rest of rig?

OCing
Please tell me about how much more of an overclock can i get per each degree temp lower? Wikipedia says each 10°C speed up chemical reaction 2x so does that mean Electromigration too?

What do you think about watercooling with peltier aka ThermoElectricCooling?If watercooling Id use Isthiscorrect? user's idea https://bitcointalk.org/index.php?topic=420000.msg4565613#msg4565613 to hang it in air using strings through motherboard screwholes for best air access?and maybe an external airfan/s ?
How much pure does water have to be to be safe?
I found phase change coolers are too expensive right now

What is important for scrypt?
Memory or Core  MHz?
Can I lower the other if only one is needed?

Does mining use any SYSTEM RAM (apart from Reaper software)

Yes/No
What motherboard components affect gpu and stability?
North/South|bridge?
CPU,RAM whatever?

Waranty
What should I watch out for?

I just made account for this:
After 2 DAYS of researching I still dont know

What is needed for scrypt mining?
core or memory?
if only one can I underclockvolt the other for lower temp?

About how much better overclock will I have with these three methods? (found other methods like LN2 or Helium and peltier=TEC=ThermoElectricCooling (and Sandia cpu cooler=spinning heatsink replaces fan) also Ionic Wind Cooling but that prolly fucks up your HW with charged particles=ions.Gas cooling is only a few minute option and peltier is inefficient but cheap (ebay etc) and requires something to take heat away from its other side so Ill research it more once I choose something else first.

Aircooling?
Watercooling? (Why do people use oil?)
Phase change? (prolly too expensive even when buildin it urself and so complicated)

Im asking to maximise ROI=ReturnofInvestment I will share the knowledge here please help