2 years ago I was in a similar boat to you. I had purchased a Canon 350D (XT) in 2005. I take tens of thousands of photos a year, including different genres such as children camps (charity camps), landscape, macro, wildlife and the odd sports / portrait, etc. Particularly for the wildlife, camps and sports, I felt like a superior camera (with better AF, higher ISO) would be important, particularly as I am the official photographer for the camps, and occasionally am asked to do sports / impromptu family photography.
My Canon 350D / XT had over 70,000 shutter actuations - and was still producing quality images. however I was a bit worried it might 'die' at a critical event / moment. So at the end of 2009 - soon after the Canon 7D was announced, I felt 'this is the camera for me'. I had a number of good lenses already, some specifically suited to APS-C (1.6x crop) Canon camera bodies. I've not looked back. The images I capture with my 7D are superior to my Canon 350D (though admittedly, for landscapes there's not much difference, but the higher resolution: 18MP vs 8MP is helpful). Both have decent ISO handling, though I know FF cameras of course will be much better.
But the vastly improved AF (accuracy, flexibility, number of AF points, etc) and frame rate of the 7D for wildlife, moving children, sports is truly a huge benefit. The live view is very handy - especially for macros, and sometimes for landscapes. Generally the handling of the 7D camera is far superior to the Canon 'Rebel' series, such as your XSi - also metering, etc. So I would suggest that you get the 7D. No one yet knows for sure if there will be a 7DmkII (could be, but there appears to be more talk that a 5DmkIII might be released soon...) - there is at this stage no official indication. But you might want to wait till after 2 March 2012 to finalise your purchase, as it appears there might be a Canon announcement that date.
All the best. No matter the outcome, the 7D can be a great photographer's tool - capturing great images. It is listed at a good price now... and I think the 7D one of Canon's best bargains / value for money bodies at the moment. The weather sealing, durability, quality all come together well. Surely FF have lower noise and more powerful DOF control. But a new 7DmkII will not be available for maybe several weeks / some months, PLUS the price will be a lot higher. I hope my post is helpful. Best wishes. Do let us know your outcome.