If you are seriously into architectural photography then for the same budget as the EF 16-35mm f2.8L USM you might want to consider one of the Canon Tilt & Shift
As you are based in the Netherlands I assume you bought your 400D/XTi with the standard kit lens so the EF 16-35mm f2.8L USM is buying you better quality, an opportunity for slightly shallower depth of field but no significant improvement in field of view.
If a wider field of view is what you are after then check out these PhotoZone reviews of the Canon EF-S 10-22mm f3.5-4.5 USM
, Sigma 10-20mm f/4-5.6 EX DC HSM
or Sigma 12-24mm f/4.5-5.6 EX DG ASPHERICAL HSM
Unfortunately, fast aperture and really wide angle capability make for expensive glass, even for prime lenses. As an example, the EF 14mm f2.8L USM, reviewed here
, is a dream lens at a nightmare price.
Incidentally, that review also has words to say about the Canon EF 16-35mm f/2.8 L II USM.