This sort of question gets asked a lot in this forum and others. It elicits two kinds of response. First, some people will tell you what they would buy. I don't think that should be terribly helpful; I mean, if you want to make your decision by asking for a show of hands, buy a Canon. Second kind of response comes from people will try to help you understand your options - and this is what I'm going to do. But this is always difficult, because none of us can really tell what is going to be best for YOU. Nine out of nine responses here might recommend the K20D and yet it MIGHT be true that, for you, the K2000 (for example) is a better choice or even the Nikon D60. So, take all of our opinions with a big grain of salt.
That said, let me turn to your list.
Pentax vs Pentax
One thing is easy to say: IN the list you gave, the K20D is undoubtedly the best piece of equipment in an absolute sense - head and shoulders better than the other models you listed - and it's also certainly the best deal right now. If you're leaning toward Pentax anyway, then you should certainly consider the K20D seriously. The advantage of the K20D that would cause me personally to buy it and reject the others is the presence of 2 e-dials. I will never again buy a camera that doesn't have 2 e-dials.
But that's just me, and there are things to be said about the other Pentax models. The K2000 for example can be had at Amazon.com today for $550 while the K20D is selling for $620. That doesn't sound like a big difference, but the price for the k2000 includes either (a) one good lens (18-55) plus a AF200FG flash unit or (b) two good lenses (18-55 + 50-200); while the $620 price of the K20D buys the body alone. I'm sure that the K2000 can take terrific photos, and getting a lens and a flash for under $600 is a very good deal. You may not care about the K20D's 2 e-dials or its weather-sealing.
I'm not sure how to compare the K2000 to the K200D.
Pentax vs Nikon?
As for the question of whether to go with a great Pentax camera (the K20D) or an okay entry-level Nikon (the D60), that's a tough question.
I decided several years ago to go with Pentax for two main reasons: (1) price and (2) built-in shake reduction. I have generally been happy with my decision as I've moved from Pentax model to model - but after I started shooting for money almost three years ago, I started to wonder if perhaps Pentax was still the best choice for me and I go through periodic bouts of Nikon envy. My film SLR is a Nikon and if I quit using Pentax, I'd probably move to Nikon. But I would not switch from a K20D to any Nikon less than a D90.
If you plan to shoot mainly for yourself (which does not mean you aren't very serious - just that you aren't thinking of going into business as a photographer), then I think it's impossible to beat Pentax in the value-for-price department, not just because the Pentax bodies are such a good deal (especially with the K20D selling for $620 right now) but also because its better lenses are as good as anything Nikon and Canon shooters have available and the Pentax lenses generally cost less, too.
If you think you want to do a lot of flash photography, then Nikon may be a better choice. If you're planning to build a big system and you've got money to spend on it, Nikon may again be a better choice - more options. There are also advantages to having either Nikon or Canon over anything else: these are the "default" brands, they're what everybody else uses, so it's easier to find help, buy accessories, get new lenses from a local camera store, rent lenses, etc., if you own a Canon or Nikon. There are real advantages to using what everybody else is using.
Nevertheless, at the moment I'm NOT wracked by Nikon envy and in fact I'm feeling pretty good about sticking with Pentax. I got the 40mm limited lens yesterday and I'm enjoying getting to know it. Shake reduction probably isn't as important to me as I thought it would be (although I think it matters). More important is the K20D's build quality and ergonomics. And if I did have a Nikon system, well, I'd have fewer lenses and I'd be able to upgrade less frequently because everything costs more. And I'd STILL have Nikon envy, because if I had a D90, I'd want a D300; and if I had a D300, I'd want a D3 or D700.
If you want someone to push you in Pentax's direction, read
this excellent post from Mike Johnston's The Online Photographer. He reveals that the Pentax K20D was going to be his pick as the #2 best camera of the year (#1 being the Nikon D700) but just before he announced his pick the Pentax K-7 was announced and he felt bad about recommending a camera that was about to be superseded. And note that the K20D was ALSO mentioned as one of the cameras in
his group choice for spot #3, along with the Nikon D90, Canon 50D and the Sony A700. Actually I think this latter article about his #3 spot might be more helpful to you than the former one about #2. Notice in his list of cameras that the K20D is the least expensive in the group by a good bit.
Lenses!
One last important point that new buyers often overlook. As a rule, a more expensive body won't take better photos. Just about all dslrs on the market right now at any price point are capable of taking really good photos. Check reviews at dpreview.com: nearly everything gets a very positive review. The quality of the photos isn't determined by the body (or its sensor, or the number of pixels) so much as by the skill of the photographer and secondarily, the quality of the lenses. I could take better pictures with a Pentax K-m than my wife could take with a Nikon D3. And I could probably take better pictures with a Pentax K2000 with my new DA 40mm limited lens than I could with my K20D and, oh, one of the mediocre lenses that I've occasionally had in my possession.
So as you shop for bodies, think about lenses, too, considering both lens quality and price. As I've heard others here say repeatedly: bodies come and go, but lenses are forever. I don't take that quite literally, because I've bought, used and then sold quite a few lenses. But the general idea is sound.
No matter what you choose, it will be wrong - but not too wrong. Wrong, because there are too many choices today and no brand right now is absolutely better than any other. Whatever you pick, it won't have something that the other brand had. But you can't go too far wrong, because nobody's making truly bad cameras right now.
Good luck with your decision.
Will