One question I have is what your plans are for going either full frame or a pro body. If you stay with either the XXXx or XXD series then the best lens for your money is the EF-S 17-55mm f/2.8 IS USM. It is L grade optics in an EF-S mount and I would pit it against a EF 24-70mm f/2.8 L USM any day of the week. The lens makes a heck of a portrait and landscape lens on the 1.6 crop bodies like your Xsi.
If you then also wanted to explore shallow depth of field, I would then add the EF 50mm f/1.4 USM as your ultra depth of field lens. The 50 f/1.4 will allow you to get used to practicing with shallow depth of field all the while providing a good low light lens as well.
The 85 f/1.2 is really in a league of its own, but I love my 300 f/2.8 for portraits almost as much. I also do not pretend not to love L glass, as it truly is an addiction once you shoot with it. However, you really do not need ultra depth of field for a great portrait lens. That is unless you want to stop down past f/2.8 then the L lenses will provide the sharpest pics at these wide apertures. Almost any fast prime at F/2.8 and above is going to be sharp.
If I was forced to buy just one L lens then it would be either the EF 24-70mm f/2.8 L USM or the EF 24-105mm f/4 L IS USM for their general walk around capabilities. However, I still believe the EF-S 17-55mm f/2.8 IS USM is the best option on the 1.6 crop bodies.
Below are a few examples from the 85 f/1.2 and the 17-55 f/2.8 respectively. ;
EF 85 F/1.2 L examples
f/1.2 @ 85 mm, 1/200, ISO 100, Flash fired
[expando]http://www.redskynight.net/img/v0/p1070359320-4.jpg[/expando]
f/1.8 @ 85 mm, 1/60, ISO 1000, No Flash
[expando]http://www.redskynight.net/img/v4/p245872965-3.jpg[/expando]
f/2 @ 85 mm, 1/80, ISO 1000, No Flash
[expando]http://www.redskynight.net/img/v3/p83731256-3.jpg[/expando]
EF-S 17-55 F/2.8 examples
f/2.8 @ 55 mm, 1/800, ISO 800, Flash fired
[expando]http://www.redskynight.net/img/v1/p677633630-3.jpg[/expando]
f/2.8 @ 37 mm, 1/125, ISO 400, Flash fired
[expando]http://www.redskynight.net/img/v1/p859219949-3.jpg[/expando]
f/4 @ 43 mm, 1/20, ISO 800, Flash fired
[expando]http://www.redskynight.net/img/v1/p552113384-4.jpg[/expando]