@Jim_Wygralak Your steppers might be rated for 2A or 3A, but the drivers won’t draw that amount of current from 12V. All common drivers are “chopper” drivers, essentially a step-down current mode regulator that efficiently transforms 12V into the ~2.5V the stepper needs. So the driver will only draw about 400mA from 12V for each stepper in normal use, totaling in about 1.6A needed from the supply.
Personally, i used a Scythe 650W 80+ silver ATX supply. It provides enough power for a second heated bed and was on sale when i got it. It works pretty nicely, powering the Ramps/Arduino and a RasPi from 5VSB (which provides power as long as the PSU’s main switch is on) and has the Ramps switching it on once a print is started.
For ATX PSUs, i’d recommend anything that has an 80+ label. Not only does this guarantee a sufficient efficiency, but also that the PSU is no firecracker so cheaply made that it will release its magic smoke the first time the heated bed turns on.