I think there's a good chance that SR will exceed X3's box office but still be considered a b.o. disappointment (like KK) whereas X3 only has to do better than X2.
For me, Superman Returns seems similar to King Kong ($218m domestic, $549m worldwide):
- iconic character
- based on (or following on from) a film that everyone has seen repeatedly
- successful director with a big fan base
- big budget and SFX
- long runing time
The test is whether the tv spots can sell it to the general audience.
With X3, the audience is pretty well understood. It's the people who saw X2 in the theatres plus anyone who's discovered the franchise since through DVD and tv or because they're fans of the actors. It's not going to turn in Spider-Man numbers but it should exceed X2 in the US and do even better overseas (still an expanding market and with a weak dollar, overseas rentals are worth more).
My finger in the air projections:
US 4-day opening $100m
US total $240m
Worldwide total $500m