The ATI Radeon HD 5970 is one of the longest video cards made. You will need a pc case that is over 20 inches in length from front to back, preferably closer to 24 inches in length.
The Coolermaster HAF 932 is a huge beast that you might want to consider. Ventilation, airflow, and cooling are excellent. http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=haf+932+full+tower&x=18&y=33