When I am finished with the brew and want to top it off in the primary fermenter to make it an even 5 gallons. What type of water can I use. Can I use Jugs of purfied water from the store?
Firstly, I agree with the above answers. Practically speaking you'll make great beer with whatever un-chlorinated water is convenient and sanitary. I'm just adding some more data in case you lay awake at night thinking about water too:D
If your brewing extract on the stove, I would recommend you use Reverse Osmosis (RO) water from the store. There are water stores all over that will fill up an old water container for about 25c a gallon. Since your brewing with extract, the necessary minerals are already present as they were in the original mash water when the wort was made. Some of the "natural spring" water choices are high in minerals, and could end up with more minerals than you really want in your beer.
I just make sure to star-san everything before getting the jugs filled. But if you are concerned it might not hurt to toss a campden tablet in there. Make sure you give the campden time to break down before adding to your beer or you'll kill off your yeassties.
Also: One thing that I have found useful is to cool my top off water in the freezer while I brew and use to chill down those last few pesky degrees.
you can use any water that you would normally brew with. Personally, I would boil and chill the topoff water the same as I would the Wort for sterilization purposes. You can probably get away with any of the packaged water from the store without too much concern over sanitation. I'm just a bit more paranoid!