iv'e had both the 40d and the 5dc in fact i sold the 40d and replaced it with a 5dc because i wanted full frame and the better iso capabilities of the 5dc
the only advantage the 40d has over the 5dc is a better frame rate and in your case you can still use you existing sigma lenses
the difference between the 40d and the 50d isn't much ( i have one of those too
) but out of the 2 i would get the 50d and if your concerned about the lenses you already own it would seem to be a good choice as they go for not a lot of money at the moment considering i paid 900 quid for mine at launch
i like the 5dc it's a cracking camera and still holds it's own today and for the money the can be had for is still considered a bargain
the 40d is another camera that gives great results but i think the 50d is a better buy for similar money