I just looked on B&H. I see 3 kits coming in at $500 or lower. So, basically, here are your choices.
For $450, and the cheapest kit here, is the Pentax K110D.
Check it out here
Also, as a bonus, it comes with a 2GB memory card. This one is certainly the most bang for the buck as the kit lens is superior to the next two's. But, it's all about preference...
(Also, right now the K110D kit on Butterfly Photo is $400.)
And the second cheapest, at $470 is the Samsung GX-1L:
On B&H here
Don't know much about Samsung, but I believe that are basically a Pentax camera with a different body. Rebranded Pentax essentially. So, that being the case, it probably wouldn't be so bad.
The most expensive, coming in right at $500 is the Nikon D50:
You can see that here
The D50 is a very solid camera and comes with the great Nikon name. I'm not sure it's much better than the Pentax, however, and the lens isn't really as great, and it doesn't come with the 2GB card, but it's only $50 more. Many would argue it's worth the upgrade, but be sure to gauge the feel, as both of those cameras would shoot a great picture.
As far as overall in the brands... I'll basically just touch on Pentax and Nikon here.
Both brands make awesome bodies. The K10D has certainly added a lot of versatility in the ways of Pentax that it seemed to somewhat lack in the very high end as opposed to Nikon's models. As far as lenses, I know there are a lot of Nikon lenses, but, any Pentax lens ever made can work on their current cameras. And you don't even need a converter for anything post 1970. That means a TON of lenses are available for this at all price ranges. And you know that they're going to want to stick with that feature, so your lenses you'd get now for this camera would probably translate into future models you may own.
Nikon has some very very nice high end lenses, the Nikkor lenses are amazing. They also have a very wide range of accessories. There are many Nikon users, as well, so that can be an advantage because if something goes wrong, or if you want to figure something out and can't, plenty of people are there to help. Pentax is a growing name at the moment, though, so it's not like Pentax owners are nowhere to be found.
Hope this helped. I tried to be unbiased!