From sample images floating around the web, it appears that the K7 does not match the D90 in high ISO shooting. In fact, it would be safe to say that it struggles against the Canon 40D which was released in 2007 when it comes to high ISO shooting. Thus if high ISO is important to you, the K7 is not the camera.
Having said that, high ISO performance is just one tiny aspect of image quality. You'll rarely be able to tell the difference between camera models, unless you're pixel peeping (i.e. viewing at 100%). Thus most discussion about noise tends to become fairly academic.
Go down to the camera shop and have a feel for the cameras. IMHO, the main advantage of the Nikon D90 is that you'll be buying into the Nikon system. That means you'll find all the lenses and gadgets you'll ever need, and most camera shops will stock Nikon gear.
On the other hand, Pentax does make some very nice cameras with a very amazing feature set. The K7 sounds like an amazing camera. The only downside is that you'll have a much harder time finding lenses and other accessories. Up until a few months ago, Pentax looked like a very attractive option, particularly due to the cheap and high quality lenses. In the UK, the 16-50mm DA* was retailing for £450. A similar Canon/Nikon lens would have cost £800. Pentax have since updated the prices and it looks like they're doubled the price of their lenses. So that makes them a lot less attractive in my book