what kind of jack should I get?

[skymath]

I have a full-size spare tire and a lug wrench, but no jack.  What kind of jack should I get?  I figure I should get something that can both lift up the bus for changing a tire, and also be convenient for dropping the engine down when I need to take it out for whatever reason. 

Pep boys I think had a combination jack, jack stands, and this thing you lay on to go under the car.  Is there anything in particular I should look for when choosing a jack? seems like one with little wheels to roll my engine out would be helpful, but I'm basically clueless. Every car I ever owned came with a jack already

Thanks guys!
..Sky