Need Advice: K-X vs. D-90/D5000 (will be my first SLR)

I am trying to figure out the best DSLR for me. This will be my first SLR. My objectives are as follows:

1) Family photography - kids! Action and some low light
2) Landscape and Nature shots
3) Portraits

My general philosophy is to find a great value. Basically bang for my buck. I like to make purchases where I have most features available and best quality, while feeling like I found a good value/price. I would not be willing to spend twice as much for a couple small improvements.

My general understanding of top DSLR's: (please correct me if I'm wrong or misinformed!)

1) Nikon has the best image quality
2) Canon has superior mechanical quality and auto focus (speed)

3) Pentax is a contender in both areas and possibly the best value overall, less lense availibility

A few models I'm comparing:

Pentax k-x
Nikon d5000
Nikon d90 (though more than I really want to spend...maybe my favorite though
Canon t1i (last choice)

Any input is appreciated!

Jen