SP2 is a good choice and it doesn't need so much that you can't really use the machine but it must be a clean install. All those updates from times long past are horrendous performance wise, even a clean install of sp3 is pretty decent just so long you don't get on the net with that machine.
The slowest practical XP machine might be closer to 500mhz if it is only being used for general storage and period games. Back in 2007 XP and Firefox ran pretty well on my old Katami rig, miss those days 🙁
On a far away planet reading your posts in the year 10,191.