I have a Canon 40D and I always shoot RAW. I am overall pleased with the 40D except for its low-light performance. Anything at or above ISO 800 is unusable for my taste. Just to solve this problem I am thinking of buying a Canon t4i/650D, but is it any better in that respect? I understand that in-camera processing has improved, but this would have no effect on my RAW images. The big question is whether the sensor itself has improved.
There has been an improvement between the 40D and 650D, but not that great. Certainly less than a stop. You'll get a far far greater improvement in image quality by getting more light onto the sensor. There are a number of ways to achieve this:
Depending on what lens you're currently using you may be able to get a three stop improvement by switching to a fast prime lens.
Again depending on what lens you're using, upgrading to an IS lens might allow you to go three stops longer with the shutter speed.
Finally bringing your own light (i.e. using a hotshoe mounted flashgun) can get you even more...
I would only look to sensor improvements after exhausting all other options.
High ISO performance has greatly improved over the last few years but if you scrape the bottom of the barrel you wont see much improvement!
While test results for the T4i are not out yet, if you compare the T3i to the 40D at DXO lab, their scores is almost identical (54 vs 64) and looking at the low-light scores in particular, you will see 793 vs 703, showing that most of the improvement is in the low-light side of things. Put a 7D there and it gets a 66 score with 854 points for low-light which a step better. Here is the 3-way comparison.
This is assuming you are sticking with a Canon cropped-sensor DSLR. You can do much better buy going full-frame for a 5D Mark III which gets a score of 81 with 2293 points of high-ISO. A cropped-sensor camera gets at most 1183 for high-ISO but you have to go with Nikon or Pentax. Of best currently is to do both and get a full-frame Nikon with the D600 getting the top low-light ISO score. See this link for a comparison of noise at all ISOs between the full-frame 5D Mark III and APS-C Pentax K-5.
Keep in mind that DxOMark is very good at characterizing very specific aspects of RAW performance but these scores only take into account a small fraction of what constitutes image quality.
This isn't specifically a Canon example (it's the Pentax K-5), but here's a shot I took at ISO 20000 (converted from DNG):
You can see a larger version here. Of course, I did noise reduction and you can easily see detail loss, but it's a very usable shot. Heck, I did use it as part of my Project 365 back in 2010. So, the technology of the sensors have improved massively over the last few years. The megapixel race has given over to the ISO race I think.
It seems to me that the "correct" answer to this depends heavily on the brand of sensor you look at.
Canon used to have a fairly substantial lead in terms of sensor noise, but over the last few years their sensors don't seem (at least to me) to have improved much.
Sony's sensors (used not only in Sony, but also Nikon and Pentax cameras) were substantially inferior until relatively recently, but have improved a lot over the last couple of years.