I used Pentax for years, owned a K110d and a K200d. Took some fantastic pictures but eventually changed to Nikon simply because I found they performed better in low light conditions and I find it easier to get hold of lenses.
Pay attention to the lenses that are available for the body. That's the best way to decide which system to go for. After that, look at the bodies available for the system and get the best one you can afford. But, the lenses are where most of the money goes anyways, so if you have to scrimp, do so on the body, not on the lenses.
Quality lenses will last decades, you'll probably replace the body at least once during that time.
I have only ever worked with pentax, you can check out my gallery. My wife uses sony alpha. Pentax are very good quality cameras, they have good glass and decent image processors. They are slow in comparison to other brands in terms of burst rates etc. The good thing about pentax is that all their lenses are compatible, if you find a 24mm prime sigma lens from the 1970's, it will fit the latest camera they release.
Pentax makes good cameras. Their film cameras used to have a reputation among newspaper photographers as good, solid workhorse cameras. That doesn’t necessarily mean that the K-x is going to be the best one for you, though. My recommendation would be to visit a camera store that sells a few different brands. Ask to see one of each. Hold it in your hand and see how comfortable it is. Is it a good weight? Do the controls seem to be in a good spot, easy to reach? Then have the sales rep demonstrate some of the menus. Are they logical, or confusing? Ask about the range of lenses (how many are available?; are there any issues with legacy lenses?; do any third-party lens manufacturers make lenses for them?). Find out what other accessories are available, and how those compare from brand to brand. Then make your choice.