I just brewed an imperial brown ale last night, got done with pitching and cleanup at about 11pm. Pitched 1 pack of S-04 into 4 gallons of 1.099 OG wort. As of 7:00am this morning, there is already 1/4 inch krausen and it's bubbling away. EDIT: And no rehydration, either. I just sprinkle the yeast on top. Didn't aerate, didn't mix it in, just sprinkled and went to bed.
Dry yeast packets contain double the viable yeast from liquid yeast packs, and dry yeast also has the advantage of lasting pretty much forever in refrigeration. I only recommend using 1/2 pack dry yeast for 5 gallons under most circumstances, unless brewing a lager or a really strong ale as I have above, then use the whole pack.
For liquid yeast, it's usually best to make a starter, but I still only use 1 pack, unless I were to brew 10-15 gallons or more, then I might use 2 or 3. Otherwise 1 is good. The stronger the beer, the bigger the starter, or if brewing a lager then make a bigger starter.
You can use MrMalty.com to calculate things as well, but I always slide the bar all the way towards minimum packages, and round everything way down as he's overly conservative. Ignore his advice when using dry yeast.