i think the best buy in renting is the 85/1.4 or the 70-200VR. Both are super lenses that can be had here in San Francisco for $30 for the weekend or per day (sometimes less if on special). I have the 70-200 and find it great when I want to be further from my subject and I had the 85/1.4 but replaced it with the 85/1.8 so I could pick up some flashes w/ the savings. Your 50 should also be good on digital. I'd also go with others in suggesting picking up the 85/1.8 used when you find one. Great bang for the buck, much like the 50/1.8, and it's very versatile for a variety subjects and light-conditions.
I also have the 105 micro, and while i find it super for macro shots, don't like it as much for people. If you like that focal length, I'd suggest the 105 f2 DC lens, which was designed more with people in mind.
Good luck,
Brian