Based on all your reply's, it would seem there is an acceptable range somewhere in between 800 and 1600 on mid range bodies.
If you want better IQ, with respect to noise, you want to capture more light so that you have a stronger signal that stands clearly above the noise floor. For any given sensor efficiency/technology, there are three ways (other than using flash or extra lighting) you can get more signal recorded....
1. You use a slower shutter speed in order to gather the light for longer;
2. You use a wider aperture in order to let more light in while the shutter is open;
3. You use a bigger sensor area and a bigger lens so that even for the same shutter speed and aperture value you can gather more light.
Increasing the ISO does not help you to gather more light. What is does is to amplify the light you did capture, but that also amplifies the noise, somewhat, so your signal/image does not look so clean. The higher the ISO you use, the more you are amplifying the signal and the noise, so the worse things look.
Increasing sensor size does not amplify anything; it simply gathers more light, like sticking two buckets under a waterfall instead of only one. A full frame sensor has 2.56X as much area as a crop body like a 10D and so should be around one whole stop better (twice as good) at capturing more light. Thus if the sensors used identical technology for the pixels and electronics the full frame camera at 3200 ISO should look at least as good as a crop body at 1600 ISO, perhaps a little better.
Now, if big sensors record more signal than smaller sensors (providing you use a lens to suit, and make use of that extra area) then it follows that by cropping your images you are discarding some of the light you captured, effectively making your sensor appear even smaller. So, imagine you use a crop body with 3000x2000 pixels and you then crop that image down to 1500x1000. In a stroke you have thrown away three quarters of the signal recorded by the sensor, leaving only one quarter remaining with useful signal. Thus you have effectively reduced your sensor's performance by two stops and an image from that cropped section at 800 ISO would look no better than the same image at 3200 ISO if you had kept the whole frame without cropping.
What all this means is that while sensor technology improves all the time it comes at a stiff price to the pocket. However, if you can work on your technique in order to get closer to your subject and fill more of the frame in the first place then you will reap the benefits of improved IQ with no cost. I've never used a 10D but I'd be surprised if you couldn't get something useable from it at 800 ISO by shooting raw, exposing to the right, filling the frame and processing the image sympathetically.
BTW, if you had cropped that mythical 3000x2000 APS-C image down to just 1000x667 you would end up using an area of the sensor about equal to a compact camera, and we know the problems they have with noise, regardless of how many megapixels they have. So, use the lowest ISO you can comfortably get away with, fill the frame as much as you can, and shoot raw so that you can fine tune your image after capturing the shot, rather than shooting JPEG and locking in the NR, WB, tone curve, sharpening etc. within the camera.
