Which camera to choose? Nikon/Canon?

I'm looking into buying a camera.

Nikon D300
Canon Powershot SD750
Nikon D50
Nikon D40
Canon 5D mk II

If you have any of the cameras above, tell me what you like about them/different aspects, or why I should choose one out of the whole group.

Added (1). P.S. I realize these cameras are all very different

I use a D300 a lot. If I were to buy a full frame camera like the listed 5D, Mark II, I would buy the Nikon D700, also a full frame camera

I have the cool Nikon D40 so I can sync with flash at high shutter speeds, something that CMOS sensored cameras can't do.

Both brands are very different. You can to consider what you're going to use your cameras for. Canons are AMAZING at portraits. Nikon's are great for landscape, sporting, concerts, etc. I have the Nikon D5000 and its a great camera. Both brands are great, you just have to decide what the use of your camera is going to be.

I have a canon IXY and image sensor fail in due time.

I'm on my second Nikon D40.It's good, cheap (now) and fast. We picked up our first D40 with the 55 to 200 VR (vibration reduction) lens - and I use that for most of my landscapes/portraits. We use the shorter 18-55 mm for close ups and night time shots. With the new algorithms in cameras - just about anyone can take really nice photos. The D40 is light, and though some people deride the "plastic" lenses - mine have continued to perform beautifully through 27, 000+ shots. The only real beef I have with the Nikon is that its very sensitive to humidity - do not use it humid environments - Nikon will not, repeat not, honor the warranty. But for day-to-day use - I love this camera. We purchased our replacement camera (body only) from Henry's here in Canada for $267.--. Now my next choice of camera - that will be either the new Nikon 7000 or a second-hand Nikon D200. BTW a great site for info on Nikons is run by aguy called Ken Rockwell. Should you purchase a D40 - use his suggested camera settings. Nikons default out-of-the-box settings for the D40 are lousy.