You can do either, decant or pitch the whole thing. I usually put it in the fridge over night and decant the next day, but this is primarily because the DME I use is old, and I don't want to incorporate off flavors. It has been said that off flavors occur anyways in the wort of a starter due to the aeration, but I guess these get cleaned up with the fermentation of a 5 gallon batch.
Another thing to realize is that the krausen of a starter will not be like that of your five gallon batch. It will appear to be smaller and you really won't see a lot of activity unless you make a really big starter. After 18-24 hours, you should be ready to pitch. As long as your yeast is new, you should have a healthy starter. Are you using a stir plate, or just intermitant swirling? How big are you making it?