this is what I have, it meets all the specifications above....
...and is (as we say in Bristle) lush.
Just a thought. Are you sure you need such a large hard drive? 250 Gb is only necessary if you are keeping a lot of video. As an indication, most feature films will use about 1/2 a Gb, so are you really going to keep the equivalent of a couple of hundred films on it?
...a smaller hard drive is cheaper. For example, the computer I linked to ships with either 160 Gb or 250 Gb discs, there is more than £200 difference in price (though in fairness there are a couple of other differences between the two, but not many). If it turns out you do need more storage in the future, then a plug in hard drive (with another 500 Gb, say) costs about £50....
The processor speed (the one marked in Ghz) is the most important number, as this is difficult or impossible to upgrade. RAM memory (again expressed in Gb) is also important, depending on what you want to do and what operating system. More of it means you can do more stuff at once, and is useful for video/picture editing. Remember that if you do choose a Vista system, this needs more RAM to operate normally than most others. So you get more use from 2Gb on a Mac, Linux or an XP system than you do on a Vista system. In most computers RAM is easy and cheap to upgrade though....
Hope this is helpful, if dull and geeky.