How you get 48 does not matter, I normally use a 4Mhz resonator PLL'ed up to 48. I only use the 4 because that is what I had at the time I started playing with the 4550, then I found out that you can run a "standard" 18F4550/2550 at 3.3 volts and still achieve the 48Mhz with PLL. So that would imply that the slower the external the less power consumed, as with most things...