Selecting which water blocks to include is usually the easiest part buying; it simply comes down to budget. Shop around several sites that carry water blocks and decide on the best block to fit your needs. Make sure to take note of the material the block is made of (usually copper) and the barb size for your hose fittings. Several sites sell high end blocks that are made out of silver rather than copper. Despite the obvious "bling" factor that this provides, the advantages to using a silver block over a copper block are extremely minimal, so they are usually not recommended even if you can afford them.
If you're planning on including your video card in the loop, it's often a good idea to buy two blocks to cool the card's GPU and RAM separately. Large blocks that cool both at once are often difficult to install, as the height of the components can vary on each graphics card, and a poor mounting job could lead to disastrous results. In most situations, your best option is to buy a block for the GPU, and add a set of copper ramsinks if you're required to remove the stock RAM cooling.
Common block solutions