A quick way to reduce power consumption on a GX280, aside from what Steve mentioned about the CPU swap, is to run pfSense from a SSD or Compact Flash card. Run the nano/Embedded version to reduce wear on the SSD/CF card (Will they go in to sleep in pfSense? If so, even just running that with a spinning drive could spin down the drive to save power.)
Also, disconnect the CDROM, you're not using it, same with the floppy if it's so equipped. Also disconnect the keyboard (and mouse if one is connected.) All these things are small, but they add up.
GX280's have a PCI-Express slot, and often a video card occupying it. Take it out, use the onboard video (there may be a small grey cover over the onboard video port.
The down side of a GX280 is that it's (assuming you can't get a P4-M) still a P4 with Hyperthreadding. This was previous to much of any power saving features. A GX280 can't reduce the clock or FSB speed through the BIOS (well, I think there's some kind of compatibility mode that's crazy slow, you don't want that.) So, if you can't take out the CPU, you're stuck with the core of your system being power hungry.
I really should take some measurements of the systems I have hanging around my house. I'm pretty sure my old PII Celeron based GX100 is right around 35 watts, but I need to test it.