for the XSi, get the nifty fifty (canon 50mm 1.8), its only $100, cheaper if you buy it used here. the 1.6 factor of the XSi makes it an ideal focal length for portraits (50mm X 1.6=80mm)
OR you can get the nifty fifty, AND get the 85 1.8, now youre not torn in the middle!
i do have both the Sigma 50 and the Canon 85mm 1.8, though i just got the Sigma 50 about 4 hours ago so field test, and controlled indoor test are in order.
just a warning, any other lens that you are going to buy will get you into the lens addiction. i am still lusting for the 70-200 2.8 IS II but im controlling myself not to get it.....for now. 
your best bet would be go to a camera shop and play with the Canon 50 1.4, nifty fifty and the Canon 85 1.8 and see which focal length you want most.
if youre looking for a walkaround lens that embodies what you need, which is portrait and walk-around, i would suggest getting a 17-40L. just watch the sell section here and you could score one for a little bit over $500.
actually, if you head over the sell section right now, you will find the 17-40L, 85 1.8 and 50 1.4
or you can go with Canon 15-85mm. just throwing out option you could try out.
hope you get the lens that you want and need!