ram1009 :
Copper is a better conductor than aluminum but, in this application, insignificantly so, IMHO. I doubt you'd see even a degree of lower temp assuming equal core density. Also, the hose sizes are probably much too large for a computer. Personally, I would buy a well designed and highly rated pre-made system. Besdes, I doubt you'll save any money by having to buy individual components rather than a system.
Ram,
I am not meaning to flame you, but please do some research before giving out bad info like you did. The reason is that Copper and Aluminum in the same loop is a recipe for failure. Having copper and aluminum in the same loop will cause galvanic corrosion (aka. electrolysis. look it up on
wikipedia) fairly quickly (sometimes as quickly as within a few days to weeks) which over time can and will cause poor cooling & possibly leaks in the cooling system. Just look around and you will be able to quickly find what kind of damage you will get mixing the two metals (here is an example of:
nasty looking galvanic corrosion).
Also the problem with most premade kits is that they are usually mixed metal kits (copper and aluminum for the vast majority), and for the most part use sub-standard parts. Whether that be restrictive radiators, small diameter tubing, weak pumps, reservoirs that crack easily, or very restrictive radiators.
The best "premade kits" will cost at least $200US, and usually come from companies like Swiftech and DangerDen. These kits are usually made with high quality parts and use copper or brass (there is no risk of galvanic corrosion with copper and brass in the same loop) for the metal components. Along with good pumps, high flow tubing (1/2"ID), and good reserviors.
I would recommend checking out the following sites for good kits that will last a while and work quite well:
DangerDen Standard kits, custom built kits, and components for building a completely custom cooling loop.
Petra's Tech Shop Standard kits, custom built kits, and components for building a completely custom cooling loop.
Petra's also sell the following kit from Swiftech which is a great deal for veryt good performance:
Swiftech H20-220-APEX ULTIMA CPU Liquid Cooling Kit w/ Apogee XT &239.95
Another thing to remember is that alot of the first people who build water cooled system used heater cores for rad's and they worked quite well. I bet if the OP looked around there will be links to instructions for modifying the heater core to work well for him.
-ouch1