Forget any concerns you might have about DSLR complexity - you're honestly not going to find it much of a challenge, and in any case the FZ50 has the same level of complexity as a DSLR apart from the non interchangeable lens.
There will probably be a myriad of options buried away in menus and sub menus on both cameras, but those can wait. It's not a bad idea to stick the auto focus and auto exposure on while you get used to the feel of the camera and the lens range, and just shoot jpegs for a while, then start experimenting with aperture priority and manual focus as you get more comfortable with it.
I think you need to decide what sort of pictures you can see yourself taking, now and in the future, which is pretty difficult and is a good reason to go for the flexibility of a DSLR. If you want the potential to shoot in low light (pubs/gigs/dusk etc) or a proper wide angle, then the D40X is the one. If not, and you're ok with a wide angle restricted to 35mm (I personally think this is really a major drawback), then go for the FZ50 (though bear in mind that it may be replaced by an upgraded model this year, whereas the D40X is a recent upgrade to the D40).
The D40X will probably come bundled with a 27-80mm equiv zoom, but as it's a DSLR you have the option of buying an extra lenses at some point of the future, or you could try to find somewhere that will sell you the camera with a longer zoom as the kit lens at a reasonable price (80mm is ok for head & shoulder portraits, but isn't a real telephoto).
The differences in image quality as a result of sensor size will be negligible in 6x4" prints, but you'll probably find the FZ50 restricts you to small/medium size prints. Take a look at the sensor sizes:
The Nikon's is the APS-C on the right, the FZ50's is the 1/1.8" at the middle of the bottom. There's a factor of 10x in sensor area between the two. As sensor size is analogous to film size from the pre-digital era, it's bigger than the difference between an old Kodak Instamatic and a 35mm film SLR.
Bottom line:
I'd hate to see you spend £300 on an FZ50, only to find in 6 month's time that your newly found interest in shooting wide angle and/or low light and/or producing A4 or larger prints means you've got to fork out another £400+ on a DSLR.