If you want to buy a full frame camera eventually, just buy only EF lenses now. The EF-S lens line from Canon only fits on crop sensor cameras. The EF lenses will work on all 3 of canon's sensor sizes. If you want to get a new lens for portraiture, I recommend the 85mm f/1.8 if you're on a budget, or the 85mm f/1.2L if you're not. I would say get the 50mm, but if you switch to a full frame body in the future, you'll want the 85mm instead.
If you like shooting portraits and other stuff, get the 70-200mm f/2.8L. It will give you a wide enough aperture to shoot effective portraits, while maintaining some utility with the zoom.
Any of the lenses I mentioned will work with any canon camera including your XTi, the 1d, 5d, or 1ds.
While I agree with the 85 1.8 and 85L make great portrait lenses, you have to take into account the room you have to work with. Not everyone has a full size studio.
If you use an 85mm lens you will need 21 feet from your camera to the subject to get a 5'11 person completely in frame. For a 50mm it's around 12ft.
That's a lot of distance for the 85mm. I don't have that much space to work with in my studio so I wind up using my 24-70 more than anything else. I use my 85mm for head shots though. But the 85mm just isn't flexible enough for me to be my only lens.
If you're going to buy a single lens due to budget constraints, I wouldn't buy a prime. I would buy a good zoom. Modern zooms like the 24-70 or 24-105 are exceptionally sharp. Take a look at my work, if you see a soft portrait in my port, please point it out. Most of my port was shot with a 24-70.
Primes aren't for everyone.