I am in the same boat as you and after a lot of research and questions I know what im getting.
Point and shoot
+small & easy to carry arround
+simple to use, just point and shoot
-Takes ages to take multiple photo's and is slow
-Quality is generaly not as good
-not as satisfying to use
-not as many options to play with
DSLR
-Bigger to carry
-more expensive
+Nice to hold, like a gun!
+very satisfying to use
+quick rapid picture taking with no lag
+batter image quality
+Lots of options if you need them (there is auto mode)
+lts of expanshions and extra's if you wish, like lense's.
I was choosing between the camera's you said, the nikon D40x and the Canon rebel XTI 400D, but I was also looking at a third, the new Sony Alpha 100 and this is the option I chose.
Unlike the Nikon and Canon, the sony has Image stabalisation built into the camera and so it is on every lense you use, even the kit lense which the others do not have. This was a big plus for the sony. It also has anti dust features which the nikon does not have but the canon does.
The second big point with the sony is that the kit lense which comes with the camera is actually nicer and has a better zoom than both the nikon and canon camera's.
Sony actually bought out Konic manolta which are an experianced SLR camera and lense manufacturer and the sony Alpha 100 is compatable with all these lenses made by this company.
There is a smaller number of available lenses for the sony, but there is a nice selection of lenses available and as time goes on there will be more. also remember you can use Konic minolta lense's two so if you count them there are hundreds!
I wont go into everything as you can have a look into these cameras on the net. may i recommend the website
http://www.cameralabs.com as they have great reveiws and nice video reveiws also.
These are the reasons I have chosen the sony alpha 100 and people who have them have left great reveiws and seem to love this camera. The old fashion hard core DSLR users will say either cannon or nikon as they are the old school "experianced" SLR makes and there all possessed and have these makes embeded in there skull.
The best bet is to go into a shop and actually try out all these camera's.