I have a 30D, 40D, 50D and 1D3. The 50D is better than the 40D, but not by much, and whether the areas of improvement are important to you really depends on what you shoot, the ISOs you need and whether or not your glass is long enough to always fill the frame. Major benefits of the 50D are....
- AF microadjustment can reap benefits, and sharper focus will be of value to make those 15MP count.
- If you can keep the ISO to no more than 1600 then the higher pixel density should put more pixels on your subject, which will mean you get to keep more detail in your subect and/or have more cropping opportunity. However, if you're up in the higher ISO ranges and want the detail I'm afraid that NR will probably offset any detail advantage. That said, the 50D can turn in good results at 3200 ISO but the details will be lost.
- The new LCD and high resolution preview image do make it far easier to judge whether your images are sharp, when shooting. It's really hard to judge image sharpness on the 40D and its predecessors, and even on the 1D3 as well.
- If you shoot to JPEG then there are new features like ALO and LPIC that will improve the results SOOC and you also have more control over NR. If you shoot raw then there is nothing new as far as image processing enhancements go.
Here are a couple of examples from my 50D....
With 100-400 lens at 400mm and with a Kenko 1.4X teleconverter. Shot raw and processed in Lightroom....
100% crop....
3200 ISO, shot raw and processed in DPP. Resized to 25%....
If you don't need the extras on offer by the 50D then the 40D is still a very fine camera and the savings could go a long way towards a flash gun, lens or tripod.