Why does it have to be 45 degrees?
Don't forget that there are many other contributing factors to CPU temps. Room temp is probably the biggest factor, so if your room is hot, your temps will be much higher - just common sense really in that respect.
You also don't HAVE to do two loops to get good temps. You can still use two radiators, but instead of two pumps/reses etc, just use a single good quality pump, something like a DDC 18w with a good top (Petras maybe) and that will give you a good amount of power.
If you are cooling a 4870x2, I believe they pump out 289w of heat, so you will need a triple radiator if you are adding that to the loop, and in that instance, I would personally have that on it's own loop because that will affect your temps under load.
I use universal
NB blocks....EK and Danger Den do good ones (see the DK X48-T2RSB+ thread for pics) and I've only had three socket 775 boards that these wouldn't fit on and they all used the Intel hook and eye fastening.
Most CPU blocks are universal and fit on several different platforms, but I'm not sure how things will pan out with the new socket that's on the way from Intel.
Tubing is a personal preference really. The absolute ideal size is alledgedly 7/16ID on 1/2" barbs.
I use the clearflex type stuff as I find that it doesn't kink as easily as the thinner stuff.
Why would colour make a difference? I don't understand what you mean there
Acetal vs plexy - Again, it's down to looks, plexy will start to crack over time, acetal is a bit tougher but less visible.
You need to set a budget first
