Choosing decoupling caps
The decoupling capacitor removes noise on the power rail.  It also functions to shunt high frequency currents (bypass the load).  We want to damp the transient response to reduce ringing and ripple.

At it's heart, this boils down to impedance matching... good 'ole
Z=R+j(ωL-1/ωC). Here ω is the angular frequency of your noise, L is inductance due to PCB traces and/or switching power supply inductance, and C is the load capacitance (which you increased by introducing your decoupling cap).  Low ESR parts don't contribute much to the R part, which is desirable.

If you want to go down the rabbit hole, you could determine the optimal values by using a spectrum analyzer to determine the frequency response and choosing C accordingly.  This is important for antenna and RF stuff, and usually not so important for power supply filtering.

The size of the capacitor(s) depends on the frequency of noise.  Ideally, you would decouple with several capacitors of varying size.  You may target a certain frequency response.  In high noise environments such as automotive applications, it is common to use a common mode choke.
Take a look at this article and this article for more information.  

Most of the time it's appropriate to haphazardly slap down a ceramic SMD low ESR 100nF, and maybe do a parallel 10uF.

Since you specifically mentioned i7... you could try to eliminate j at the CPU clock speed and your RAM speed (DDR?).