Alien should be more intelligent than that (i.e.: than simply firing around, destroying, then taking place). Do it the other way around:
Common people on Earth one day (of 2004) realize that aliens are among us (the grey, see my pic? heheh). Politicians and governments said they are inoffensive and willing to help us. They say that there are too much problems (pollution, economical crisis, and what not), and people have been lied about them: the world is in dire jeopardy and close to the big catastrophe. They say that the aliens are willing to aid save us, and that we really need their help. So the alien now appear here and there, they erect buildings in various cities of Earth, are sometimes seen in the streets among humans, etc. Then, it appears they have got some hand in the industrial, are buying companies, etc. Soon, they pervade all aspects of western society, though they remain in very few numbers and the commo people seldom meet one personally. Only third world nations are spared the alien "peaceful" presence.
Now, this is Earth and d20 Modern (and you could use d20 Future too). How will your players react to that? IMO: it would be much more exciting than urban arcana...
--------------------------------
EDIT: Sorry, but i am responding to this thread believing it to be another one of this forum

Well, at least I am not too far from the subject.