I have the 350D & can highly recommend it as a nice bit of kit that will give great results for the minimum amount of money spent.
The 5D though is in another league & should produce even better results but as always its not so much the equipment but the person behind it, depending on skill level you can still get stunning results with a 350D, ive sold some photos using my setup & its a great camera, but I would imagine (as I dont own one) that a 5D would just have the edge when it comes to using the camera, its like a rolls royce in the camera world.
Both cameras have the capability of producing top images but are really in 2 difference brackets, the 350D is without any doubt the best camera around for the price, value for money & image quality is right up there, the 5D on the other hand is altogether a much more expensive beast requiring even more expensive lenses.
I would say if anyone can comfortably afford the 5D with really good glass then go for it, I was going to buy the 5D instead of the 350D but when I worked out my finances it was just too much of a stuggle to get the money, one day maybe!
Nick 