+1 to Henning and the Mr. Malty pitching rate calculator.
Or, by hand, 0.042 x volume in gallons x gravity points.
0.042 x 5 x 52 = 11 grams
Behind the scenes...
You need 750,000 cells per milliliter per degree Plato of wort.
750,000 (cells/ml-Plato) x 3,785 (ml/gal) x volume (gal) x gravity points / 4 (gravity points / Plato) / 17,000,000,000 cells per gram = # grams.
Viability can range anywhere from 10 to 20 billion cells per gram for an ale yeast depending on age and storage conditions. Now, if you want to just sprinkle the yeast on top of the beer, it could be as bad as 5 billion cells per gram of yeast. You could need close to 40 grams of yeast this way. Those extra yeast? They're just dead yeast sitting in your beer. I strongly recommend that you rehydrate your yeast according to the directions on the Danstar website. Cheers!
http://www.danstaryeast.com/rehyd.html