Wow man, keep us updated on that big ol' ship, I've been eyeing that baby since I first saw it.
We use a lot of tiles and some card buildings. I've gotten tiles from most of the sources mentioned above, but I've made most of my favorite ones by simply finding a nice texture, pasting it into Powerpoint, then setting a one-inch very thin grid and setting the slide to 8x10 inches. Presto, instant cool tile with no problem making them. I did our modern roads/highways/parking lots that way.
One thing we also do is that for our big "battlemat" we took tiles and then used spray adhesive to affix them to large pieces of foamcore. This way they are light, stiff, fairly durable, easy to plunk down on the table, and dont have the annoying habit of not fitting together or bending/warping on the edges like card often does. We dont use any cardstock miniatures, I prefer things 3 dimensional, but I've gotten some nice 'casualty' minis from some historical miniature lines pretty cheap on ebay and painted them up for that purpose.
I've also used Microtactix for their city sets and their Mad scientists lab, which were fairly easy to put together and REALLY dress up a Modern city scene so well. We've used some games workshop Mordheim/Warhammer card buildings too. Throw in my road tiles, some cheap toys/accessories, and some cheapo scale cars with the minis and you have a good city scene.
A couple members of our group have gotten into Hirst Arts so the cardstock buildings and whatnot are on a downturn in use, as their stuff is so much nicer.
There are some pictures of our tiles and setups in the miniatures thread in my sig if you'd like to see our stuff in action.