i would suggest a 50mm f1.4 as well, however, i would suggest the Sigma 50mm simply because it's bokeh, is sublime. Regardless of what anyone says, portraits vary too much for one lens to be the answer, i've used anything from 20mm (on full frame) to 135mm (also full frame). It depends on the subject and what you're trying to accomplish, If it's a typical head and shoulders shot, then 50 / 85 should work, but be aware that at large apertures, you need to be steady so that the thin DOF falls where you want it to.


This was taken with a 18-55 on an xti at @18mm (28mm equivalent)





I got close to her and composed with some of the environment.


Here is a 20mm shot on an F100 (full frame film body):





Much looser composition but again, it may fit for the shot i was trying to get.


This was with an 85mm @f2 on a crop body (130mm equivalent)





Much tigher crop, but it also empahasizes the reflections and the brand of the glasses.





All of these are uncropped so you get an idea... There is no perfect portrait lens, it really depends what you need to do. However, you can't go wrong with a 50 or an 85. Your 17-55 is very capable already, so maybe an 85mm will give you more versatility.