The MOBO defaults to 1.8v because that is the stock DDR2 specification. Any voltage beyond that which is needed for OCing, you will need to set manually.
If you want to OC, get the 2.1v version. Your links don't work for me but if I was making recommendations, get PC8500.
First of all, LINKS ARE FIXED, IT'S WORKING NOW
Second, say if i buy C4 version - that runs at 2.1V with 4-4-4 timings... Now when i install the ram, i assume it will run by default at 1.8V with 5-5-5 timings, am i right ?
Now, if i want to run this ram (C4) at 1.8V and at 5-5-5 timings throughout and would never like to overclock is that okay and safe and would pose no problem? I am asking this as "C5" stocks are not available at the moment where i live..