The 40D would be another one to consider, I've used it extensively for wildlife and found it to work great (And the 40D is extremely similar to the 50D with the difference being resolution and microadjust). I currently own both the 40D and 7D so I'll share some of my observations between the two. My thoughts will more than likely hold true for the 50D as well, because the two have the same autofocus system and similar high ISO performance.
I've found the extra resolution is nice on the 7D, although unless you print large you'll not need it. So I wouldn't put one camera over the other purely based on resolution. Autofocus is noticeably better on the 7D and I find that I trust it a bit more than the 40D. The viewfinder on the 7D is significantly better than the 40D, the 40D's viewfinder looks small and dim in comparison. There is a slight difference in feel for the grips of both cameras, I don't find either better than the other, just a bit different.
As for ISO performance, in my experience I find that for the 40D ISOs 100-800 are extremely usable with a noticeable loss of detail at ISO 1600 but it is still very usable. For the greatest detail I use 100-400, as 800 there is a bit of a detail loss compared to lower ISOs although it is well within my tolerances. With the 7D I find 100-1600 are extremely usable, with 100-800 retaining the greatest amount of detail. ISO 3200 is usable and is just a bit worse than ISO 1600 on the 40D. The ISO difference is about 2/3 of a stop to 1 stop in my opinion.
The burst rate is another difference, the difference between 6.5 and 8 FPS is noticeable, although either will be a hug leap compared to your XS. I've never felt that 6.5 FPS was holding me back though, so I wouldn't get the 7D solely for that feature, but it is nice. And that's really all the noticeable differences between the two from my experience of using both extensively. As far as your situation, I'd recommend getting a used 40D, as they can be had for less than $600 now and the price won't really get that much lower anymore. Perhaps it'll drop $1-200 more in the next year or two, but that's a lot less than the 50D or 7D will drop in that same time period. So if you find that you need something more, you could sell it off with little loss.
So IMO I'd recommend the 40D and use the $1200 that you saved over the 7D to get a nice telephoto prime and perhaps a teleconverter. I'd probably get the Canon 300mm ƒ/4L IS and Canon 1.4x extender, the 300mm is a very sharp lens and you should notice a big difference compared to your Tamron 200-500.