What's the diameter of a headset...the part that actually presses into the tube?
I just got a ATB frame and the inside diameter of the headset tube is 1.45". Do I get a 1" or 1 1/8" headset?
That is a VERY good question. 1.45" is about 37mm. Hm. Does you bike have a threaded steerer on the fork or a threadless steerer? If it is a threaded steerer you will want an 1-1/8" headset. If it is a threadless steerer you will want an 1-1/4" headset. IMPORTANT to note that 1-1/4" threadless headsets are not very common except on tandems.
Easy way to tell if it is threaded or threadless- Can you raise or lower your handlebar stem just by loosening a single bolt on top of the stem? If so, it is threaded.