The advantage of running the PIC18F4620 at 5V is that it can be clocked up to 40Mhz as opposed to the PIC18LF4620 which has an upper limit of 10Mhz at 3.3V. The disadvantage of mixing 5V and 3.3v tech is that you need two voltage regulators and the extra level shifter IC(s). The route I would take is to find a 3.3V PIC ( PIC18F45K20, PIC18LF4620, etc) that you can clock at whatever speed you need and eliminate any extra parts required.
You could also look at the PIC18F97J60 and other PICs which have built in ethernet and you won't need the ENC28J60 at all.