I shoot a lot of pictures in difficult light on a T1i (my main and only Canon digital camera body).
My experience is that you want to avoid ISO 1600 if at all possible, and even ISO 800 is usually notably poor. It's best in situations like these to just get the shot you need, setting the ISO manually as high as you can tolerate, and then push up the brightness later if necessary. I use Av and Tv modes almost exclusively.
It's a nice camera for shooting things one-handed when need be. I've gotten pretty proficient (or maybe just lucky) at just aiming the camera somewhere without looking through the viewfinder and actually capturing what I'm pointing at. Did this through a windshield, while driving, to capture some balloons that escaped from a car dealer recently (yes, I was at a stoplight at the time, and no, the pictures weren't perfect but more than decent enough given the circumstances). It was a four shot burst and I was on target all four times. The camera had to autofocus on the clouds, too. Enough bragging though...
My personal feeling is that I should buy the best lenses possible for the camera. I would enjoy a full frame camera, although some of the lineup is tailored for a crop body camera better than it would be for 1.3x or full frame cameras. For example, the famous "nifty fifty" lens is one of my most-used general purpose lenses but it would go from a short telephoto "portrait" lenses (good for slightly tight landscapes, individual people shots, "portraits" of various things on tables, etc.) to just a normal lens. On the other hand, on full format I would be able to make use of the TS-E 17mm f/4L's curved front to capture even wider shots than I do now, or I could get the somewhat easier to protect 24mm f/3.5L and still shoot wider images (at a wider aperture, too). My TS-E 90mm would more or less take the place of the 50mm (which is currently about equivalent to 80mm) and although I would miss the far increased telephoto it would be much easier to use for general purpose product photos. In any case, all of these perform great on the T1i.
Where the T1i runs into trouble is (aside from low light) using slow lenses (telephoto zooms especially) on the camera. I have a Sigma 120-400mm, and there is a more or less equivalent Canon 100-400mm L. Both have maximum apertures that range from f/4.5, which is slow, to f/5.6, which is the slowest a lens can be on the T1i while still autofocusing. The Sigma struggles to achieve autofocus very often, and it still isn't "long enough" for many wildlife photos (birds mainly). It only does really well in good light and when the target is close enough to fill the viewfinder. Unfortunately, to get an even longer perspective you will have to spend a lot of money. For the most part, though, this is somewhat rare, and the f/1.4 lens almost never misses autofocus (it's even possible to autofocus at night in limited situations, and when it's completely dark out I can manually focus and review the shots on the bright back screen).
Movie mode - the 1080p 20 fps mode is an oddity; the 720p 30 mode on the other hand seems useful (though the framerate, last I checked, was still exactly 30fps, which is useful for computer viewing, but is different than the "standard" 29.97 fps rate of pro video). It won't replace a camcorder but with the appropriate lens you can do magical things with it. From what I have seen, the newer Canon DSLRs offer 720p modes at only 50 or 60 fps which is laughable for most users; it eats your runtime right up. The movie mode is useful for occasionally putting on a tripod and pointing at some still life or pointing out a window, and not much else unless you are extremely dedicated (some people have managed to shoot music videos with lenses not completely mounted, while still getting decent results; my hat's off to them).
Of course, you may have hinted at your budget by mentioning the T1i as your first choice of a DSLR. I hope that you can use the camera for a long time and invest in great lenses.
I have held a 50D, you won't likely get much out of it that you wouldn't get out of the T1i. The 50D requires Compact Flash, which is going by the wayside. If I were going to decide to get an XD series body, it would be either the 60D (better overall, but cheaper!) or a 7D (no twistable screen, but still the holy grail of APS-C cameras in Canon mount).
The bottom line is that of all relatively current APS-C cameras, be they the T1i, the T3i, the 60D, or the 7D - even the T3 for that matter (which I would pass because of the insanely cheap looking grip which I wouldn't trust) - all are going to perform relatively similar to each other in terms of image quality in usual situations. Unusual situations would be where autofocus is a primary concern (the 60D and 7D get the best marks by far) or when you are trying to pull small areas of an image out when your lens isn't long enough (but this is a bad situation for any camera). From the results I have seen, the leap in visual quality between the T1i and the next generation of 18 megapixel sensor cameras is there - but it's not a big enough leap to upgrade my camera body. However, if you can find a deal on one of the newer cameras, you will probably be better served. One of the reasons I stick with my T1i is because I don't like to waste money on new cameras without there being a really good reason for it - I like to skip at least one generation to save money. There is no reason to expect that the jump from the 18 megapixel sensor cameras to the upcoming cameras will be much farther than the leap from the 15 megapixel T1i and 50D to the 18 megapixel series, and if you buy a cheap T1i now you might find yourself looking at a new camera relatively quickly and feeling that you are stuck with an older, cheaper camera which doesn't compete as well.
But, like I said, when you look at the full picture, even the T1i does very well in most situations.
The question you should ask yourself is what sorts of situations seem most interesting to take photos of, for you. This will help you select a good camera and determine what camera features aren't essential for you.