The recommended voltage specified by the RAM manufacturer is the voltage at which they are satisfied that the module will be stable and perform as advertised. Very often the voltage supplied by the motherboard/PSU will fluctuate slightly above and below the nominal figure (in your case 2.5 volts). Anyhow, if Crucial recommend that module for your motherboard, and noting that it says "Guaranteed-compatible memory upgrades for your HP - Compaq Media Center m1180n Desktop/PC." at the top of the page, I would just go ahead.