okay...
1) copper dissipates heat better. period. abut twice as well as aluminum with gold somewhere in between. there are only 2 things better than copper at conducting heat... silver (which is about 10% better) and diamond (about 10x better). heatsinks are made with aluminum cores to save money. that is all.
2) having the fins be amalgamated with teh rest of the heatsink is best. if they're seperate and attached, you have to worry about air being trapped in microscopic gaps and about heat getting across. if it's all one peice of metal, it's the best.
3) if i were you, i'd make a copper waterblock for a watercooling setup. cuz they're pretty simple and also pretty expensive. i threw together a crude pic to show kind of how you'd want to design a waterblock. notice how the cool water comes in right in the middle on top of the core, where it's hottest. then it circles around through a wiggly path that will allow the water to extrude as much heat as posible from the copper. the pins help with that, too.. though i have no idea how you'd do that in shop class. if you want more help, i'd be happy to make more detail pics of different designs.
4) adding ethanol won't make the water colder at all. you can't add anything that will do that, unless you have a constant supply of it, like liquid nitrogen. the best you could use is distilled water (pure, no minerals) and a single drop of soap or "water-wetter", both of which will break the serface-tension of the water and allow it to flow more freely.