The main page has a complete section on watercooling articles. Depending on what type of system you want to have, case mods can range from nonexistent to very major.
I have a homemade system with a large radiator, a homemade reservoir, a custom waterblock, and a very modified case to fit it all in. Most people could go with the complete systems from Swiftech, Danger Den, or BeCooling and be quite happy. It's worth your time to read the articles on the main page. Depending on your budget and handiness with tools, you might be able to do it fairly inexpensively.
I built my own watercooling system for under $100. I used another old case to mount every thing in for the reason in case the thing desided it wants to leak it don't get my board.(unless it is the block or the 6" of tubing in the main case) Also so it would cut down on noise and be able to have more coolant.