Ok, I'm probably going to set the cat among the pigeons with this post but here goes...
You don't
NEED a 5D for wedding or portrait work. Yes it has incredible detail with outstanding noise control but your clients can't tell the difference between a well exposed and processed print from a 40D, a 5D or even a 1Ds MKIII.
Your clients don't look at a print and worry about the colour balance being ever so slightly off, not quite enough DoF, barrel distortion, etc. They see memories of people they love filled with emotion and that's what is important, not the technology behind it. I've had wedding clients ask to see every shot I've taken, even the OOF ones just in case there's something special in one of the shots I'd normally reject for being crap technically.
The forums are filled with great people all keen on photography and, let's face it, we're all a bit geeky about the technology. We pixel peep and discuss noise on 100% crops, the best way to sharpen for web of print and so on.
How often do you see a post in the photo sharing section where someone asks what camera was used, what lens, exif, etc. That tells you everything you need to know, if the geeks here can't tell immediately then the REAL difference between bodies, brands, etc. isn't worth worrying about. In terms of what you need to supply to your client a 5D isn't going to take a better picture than a 300D.
So forget about image quality between the two and look at other aspects. Would full frame be useful for the extra width - have you found on occasion you needed to go wider and if so has it happened enough that you think full frame would be a good solution? Or would a wider lens do just as well?
Or would having two bodies with different crop factors be a pain? Extra lens swapping at crucial times because you really want the 50mm on the 1.6 for the extra reach?
How you work and how the 40D or the 5D fits into that workflow are points worth considering. As a business cost is always a factor, would the extra for the 5D be better spent on another flash, a lens upgrade, etc.?
Or the other hand you might just WANT a 5D for all the geeky reasons, same as well want the latest gadgets and gear. Of course there's nothing wrong with that either