While the IQ of the 5DII may or may not be noticeably superior to your XSi, the fact of the matter is that the $1800 price tag isn't justified by the IQ alone. You're getting significantly better build quality, better controls, and most importantly a full frame sensor. The full frame sensor allows you to get shallower DOF (very useful for portraiture), has larger pixels which capture more light and less noise, and in my opinion really makes Canon's L zoom lenses shine. I recently upgraded from a 40D to a 5DII. My lenses included a 24-105L, a 70-200 f/4L IS, and a 50mm f/1.4. The zooms are both great glass, but 24mm isn't really that wide on a crop body, and the 70-200 was usually too long for portraits, my primary type of shooting. On my 5DII, I now have a great wide angle to mid tele for a walk-around lens as well as the perfect portrait lens in the 70-200 (it has lived on my 5DII).
Do I notice an improvement in IQ from my 40D? Yes, though in many cases nothing huge. However, the combination of the controls, shallower DOF, and the way it works with my lenses makes it worth every penny to me.
Honestly, if you're a self-professed cheapo whose only concern is IQ, your best bet might be a classic 5D, as that has to be about the best bang for the buck out there in terms of IQ.