I don't have any experience of the 50D but unless it's ISO range is drastically higher than the 40D I think that to expect some noise at ISO1600 is fairly reasonable. To a greater or lesser extent, noise can be greatly reduced in software but if yours doesn't allow for this you might (after conversion) want to try Noiseware Community Edition which is a very nice piece of Freeware from
www.imagenomic.com . This, to date, has satisfied my needs although there is a Pro version - at cost of course. Another couple of options would be to buy a Nikon D3 (only joking), buy a faster lens not to mention adding more light if you're in a position to do so in an effort to bring down the ISO needed somewhat.