Wow, nice catch, dell definately has a typo, it's 400 Mhz probably, because how can a 533 Mhz be also 1.8Ghz as well (the clock speed should differ) it should probably be more. Nice one, I think dell was trying to trick some people. It's a good thing that you decided to come to syschat, after what dell did to a lady (sold her a server to connect two computer, instead of a cable) I think I'm starting to worry a little bit as well...
The reason why the prices are different because the first two are under the employee purchase program that's all so the prices are lower (interesting to see that the prices aren't all that low...) so yes you are right the features are the same, the only thing that differs is that it's under the employee purchase program...
By the way if you are buying a dell don't forget to add coupon codes and all that stuff (I think you can do that, I don't know about the UK..) and all that stuff... wouldn't want to pay extra for something unnecessarily..
You can read more about the USB ports and so on
over here and look under the
tech specs area as well... and the battery should last about 2 hours on so (from general experience)..