whr-g125 & whr-hp-g125 are the same internally. They have different antennas. Both have the same FCCID FDI-09101584-0
They are powered via a 3.3V inline wall-wort, in the same style as the whr-hp-54g.
whr-g125 has a non-removable 2dBi ant. attached via coax to the pcb.
whr-hp-g125 has a 4 dBi ant. with a rp-sma ant jack that is attached to the pcb via coax. The antenna pictured in the FCC pix appears identical to the antenna shipped with the older whr-hp-g54.
4MB flash & 16MB ram remain unchanged from the older whr-g54 series.
The new models use a Broadcom BCM5354 chip where the whr-hp-g54 uses a BCM5352.
"BCM5354 Adds 802.11g Radio - Also in November, Broadcom announced its BCM5354, an upgrade to its BCM5352 802.11g router-on-a-chip. The new chip, which is already in full production, adds a direct-conversion radio to its predecessor. The resulting product combines a MIPS CPU, a five-port Fast Ethernet switch, and all 802.11g functions except for the relatively low-cost RF front end."
shows it running at 240MHz. The older whr-hp-g54 runs at 200MHz.
From the FCC site I see there was a change made in the application. First application date is 03/26/2007. A "Class II Permissive Change" has a 04/12/2007 date.
The changes: WAN jack changed from Black to Blue. A heatsink has been added to the Broadcom BCM5354 chip. The AC power adapter specs were changed from a universal 100-240VAC input & 3.3V,2A output to a 100-120VAC input & 3.3V, 1.2A output.