actually, you can't go wrong with any brand. They all have their respective pros and cons. I actually like Pentax. The K20D is a great camera. Lots of great glass for them, although they aren't as "mainstream" as nikon or canon.
Here are a few things to do research on and think about.
Like others have said, you buy into a system, per sé. Only Full frame cameras (5D/2, 1DS/II/III) are the only cameras that are NOT compatible with the EF-S line of lenses, as they are for the "crop sensor" cameras. I only know canon, but Nikon has the same kind of deal.
So say you buy Nikon Dxx, and then you buy some glass, you are more than likely to stay with Nikon. As you get more and more glass, you will then come to a point where you will have a "system", so to speak. Switch to another brand at that point just kind of becomes a hassle, as you have to sell all your gear, and then replace it with the equivalent Canon or Oly or whatever you decide on.
I would honestly buy whatever camera you want. You buy a base, "bare bones" body, and then you will outgrow it quickly. I have been shooting for a long time, and i am fine with the xD bodies of Canon, although Full Frame would be great, just don't want to spend the cash on it at this time.
I think 4 fps is perfect for every day, all around use. You can pick up a used 30D body for about $400 with low clicks. Or a Nikon D200/300 for the price of a D80 or D90. There is nothing wrong with buying used. CHeck out
http://www.fredmiranda.com . They have THE best B&S forum i have seen. Seriously, i would go used. Then when you want to upgrade, you can sell and not lose your ace on the body.
Glass holds its' value, about 70% or so. And don't worry if the camera is a lower model camera. A lot of sales people will try to push a higher camera onto you, as i experienced yesterday. An XSI is perfect. Same AF system as the 30/40D, but not quite as a fast burst rate, etc...
And while megapixels are great, i think on some cameras it is just a gimmick. The difference between 2 MP is not going to be make or break for me. MP count is great if you are blowing up images 20x30, but how many of us have even done a 10x15 on a regular basis. I have never blown any thing up over 8x10. Sad, i know...
Just do research,
bring in a CF/SD card into best buy and shoot with different cameras. Then view the info at home and see what the differences are. I know shooting indoors with a kit lens on auto isn't going to be scientific, but it will give you an idea... Buy what you want. We can all sit here and give you info and tips, but only you know how far you want to take this hobby....
http://www.dpreview.com is great, BTW.