Currently have 2 loops, 1 cooling 2x 260's oc'd, and 1 cooling i7 920 oc'd at 4.0. cpu has 240 mm rad idling at 38-40C while gpu's idle 45-50C with the same, 240 mm rad. Atm i cannot afford a fourth pump, but id like to add a third for a backup, so to beable to backup both pumps with 1 extra pump ill need to make it 1 loop total... if doing so ill still keep 2 reservoirs... but ill add 3 more 120 mm rad's makeing it a total of 7x 120 mm rads, so my QUESTION is... is it likely my temps will stay the same combining loops or more likely drop adding more rads.
Idle temps aren't of much importance, what are the load temps ?
Personally, and without having much WC experience, I'd leave the loops seperate if I already had them set up that way.
What do you mean by a backup pump ? You have some way of having the second pump kick in if the primary fails ?
Your other post states that you believe a pump to be failing after 2 yrs., what makes you think that ?
Don't know what pumps you have but most that I have seen are rated for 30-50k hrs. MTBF, meaning an expected life of roughly 3-5 yrs.
Everything I have read states that stacked rads don't perform very well. You'd be spending money and adding restriction ( although with 3 pumps I guess that probably wouldn't be a problem ) to the loop for little or no gain.
There are people here with more experience , maybe they'll chime in.
Most pumps are extremely reliable 5 years+. The liquid in WC loops are particularly clean compared to intended application of most industrial pumps such as use in professional aquarium filters. I never had an aquarium pump fails on me. I have been using some for 10+ years and they pump hard calcium rich water with waste every single day without end. The pump itself cost as much as a MCP-655 so I won't be worried.
As for idle temps. It really depends. You should check your water temps first. Your water can only ever be as cool as the air being fed through the radiators. If your water is already close to room temp, adding more radiators won't reduce temperatures. Component temperatures are always higher since heat is being produced there. Your 30c-40c idle seems accurate, the near room temp water can only remove heat so fast. If you want lower temps, try an aquarium water chiller. They are quite loud though and produce quite a bit of heat.