I used one of the options shown in that thread -- the one with the copper reducers -- and it worked great on an old Gitane frame. I felt like the reducers made a difference. I discovered after pressing in the new ISO headset cups that my frame was JIS in this regard (or possibly French, but I doubt it based on the non-French bottom bracket). Anyway, it's only a 0.2mm difference on the cups and I had one of the low quality frames Burton mentions, so I seem to have gotten away with it. I had to get the fork shaved a bit to fit the ISO crown race.

You could have ISO, JIS or French dimensions. If it's French, then the cup size is the same and you're using a different fork, so I think it's practically the same as ISO (assuming that's what your fork is). If it's JIS, then it's probably a lower quality frame, so you could probably get away with just pressing in ISO cups (although ideally you should get it reamed in this case). If it's ISO, then no worries with most off-the-shelf headsets.

A good quality caliper along with Sheldon's cribsheet should tell you what you need to know.