A very good question. As others have stated, I wouldn't count on the 7,2 from 2003 being compatible with the 970fx CPUs from the 2005 7,3 model - but I can't say for sure, as I haven't tried this myself and I have not held these two CPUs side by side to determine if they are physically a match or not. So, buying the parts expecting this to work would be a gamble.
Around 2017, I performed a 2003 7,2 DP 1.8GHz > DP 2.0GHz upgrade and a Late 2005 11,2 DC 2.0GHz > DC 2.3GHz. Both upgrades provided a decent enough jump to make the exercise worthwhile.
The 2.3 DC ran great (calibrated perfectly, ran cool and quiet) for about 4 more years after this upgrade, until eventually, bank by bank it lost the capacity to use memory modules (of any spec). A few months back I completely decomissioned that G5, and it's role was re-assigned to an existing C2D 2.0GHz 2009 Mac mini with 6GB of RAM and a 7200rpm 500GB spinner.
The charming little incognito mini blends perfectly with my PowerPC hardware, runs Leopard natively and handles practically all the same apps the G5 did (Pro Tools LE + Ableton Live DAWs, Propellerheads Reason, and a ton of Native Instruments plugins, FX VSTs, etc) - although not PowerPC, it was a perfect "cross-grade" from the G5. The lower-spec C2D actually performs notably faster than the G5 did, with far more efficieny of power and is practically silent - plus it didn't cost me anything to dust the mini off and migrate the G5's HDD to the mini to get back up and running with all the same software config.
So energy consumption of this unit went from around 250 - 300w/hour to ~12w/hour for the same software setup.
Nothing against the G5. I have 4 of them (incuding the failed DC 2.3) which unfortunately get next to no run time. I am currently posting from my 12" PowerBook G4 though - so I haven't forsaken my PowerPC roots just yet
And I still have a (noisy) Quad 2.5GHz Late '05 G5, which refuses to calibrate, despite all my efforts to service the liquid cooling system, renew thermal paste, etc. I will get to this one day. Maybe I could source another 2.0GHz DC CPU and try the air-cooling "downgrade" as others have suggested to give it a chance to calibrate, stay cool and quiet.