I found a good set of web pages on aluminum casting that describe the sand casting and lost foam casting processes (as well as other interesting info like how to build a forge at home). You can find them here:
Casting Aluminum

Don't know for sure which is better to make multiple items of the same design, but I would guess it would be sand casting since using that method you get to keep the original object you are making a sand mold of. Whereas, it lost foam casting, the object you've modeled out of foam gets burned up. The method you pick might also be determined by shape and size of the item you are casting.

[Note: I'm no expert in casting. My only experience is taking the aluminum casting SBU.]

Mike