My cat just fell out the window...

If you have to move your cat then do the following (it'll cause lots of stress).

Leave a travel box in the cat's favourite spot in your house for a couple of weeks, put some treats in there to encourage the cat to be curious about it.
Pack up the cat for travel in the box a couple of weeks later and pack it's blanket/litter tray/sleeping cushion/etc.
Upon arrival, set the cat up in a small, quiet room, away from all the other cats and smells, etc (put the litter tray in one corner and the eating area in the other furthest corner).
Spend lots of time with the cat and hopefully it won't be too stressed as it will view this new room as it's home.
The introduction of the cats to each other will not go down very well, you can do this in many different ways (travel boxes nearer and nearer to each other, a new "mutual" room that both cats aren't used to, introduce the more aggressive cat into the other timid cat's area as a fight is less likely, etc), if you plan on only having your cat somewhere for a couple of weeks I'd keep them all in separate parts of the house for ease, at the very least I would let the cat get over the initial journey/new home before adding further stress with cat introductions, etc.

Or find a cat like my Tommy who regularly travels between London and Devon. Goes outside at both places, and absolutely loves travelling in the car - to the point that most mornings the other half has to stop him from climbing in when she goes to work!