The wattage will remain about the same. Converting voltages is much easier with AC, since the current is alternating and you can use a transformer to do it. For DC, it's much harder to convert voltages, and less efficient.
Watts are the same regardless if it's AC or DC. Watts are always volts x amps. If you look on the back of many appliances with worldwide power supplies, you'll see something like 110VAC, 1 amp/ 220VAC, 500ma. In both cases the wattage is the same, since you doubled the volts and halved the current.
For your computer, the wattage won't change very much when you change power supplies. If it's using 77 watts normal, 132 watts under load, it's going to use about the same even (or even more, depending on the PS efficiency) with a DC power supply.
A 90 watt supply will not work. You're still going to need a much bigger one than that. A P4 2.4 ghz at full load uses about 75 watts just by itself.