If you have a double, you have a 53-tooth chainring. If you have a triple, it's a 52.
There are subtle but meaningful differences between front derailleurs meant for doubles and those meant for triples. They will work either way, but if you have a triple you'll get better results with the triple FD.
Tooth capacity isn't the important issue. Choose the FD based on whether you have a double or triple crankset. If you're buying a new one, there's no reason not to.
You can replace a Tiagra triple crankset, front derailleur, or bottom bracket with 105 or Ultregra triple parts, in any order, in any combination. If you are changing to a double, you will need to replace the crankset and bb together.
Training: 2002 Fuji Roubaix Pro (105 triple)
Commuting/Daytripping: 2001 Airborne Carpe Diem (Ultegra/XTR, touring wheels)
Commuting/Touring: 2000 Novara Randonee (Sora/Tiagra/LX, fenders, lights)