The Pentax K100 and K100D (the same thing but with built in image stability) are great cameras. I’ve owned a K100D. However, if you get bitten by “the bug,” you’ll probably want to switch to a Canon or Nikon system because of all the extras you can get for it. I would say go ahead and just start with either a Canon or Nikon camera now so you don’t feel you wasted money, and so you won’t have to learn different controls.
I would say look for a Canon 30D (review). They run about $400–500 in the used market, but I think in the long run you would be happier with it than a Pentax camera. If you want to go a bit cheaper, the Canon 20D is still a good camera for learning and because it’s a generation older, it’s a little cheaper.
Also you can go for a Digital Rebel but the controls are a tad different (like holding down an extra button for changing aperture) and they feel a bit less durable. The K100 does feel more like a DSLR should compared to them but the 20D/30D feel even more like a DSLR should than the K100. However the Rebels can run about $100 cheaper in the used market.
For a lens, you should look in to a Canon 50mm f/1.8 which runs about $90 new. This lens will allow you to control the depth of field like crazy and with it you will learn loads more about camera control.
The setup of the 30D and the 50mm f/1.8 is probably more than you were wanting to spend, but if you want a DSLR to learn photography or become better at it, I think this is the route you should go. Otherwise you’ll end up getting a DSLR that you might just use a point-and-shoot with a bigger sensor.
If you think you might be more of a Nikon person, look for a Nikon D50 (review) that goes for about $400–500 used as well and a Nikkor 50mm f/1.8 for about $150 new.