I typically go with a 2-year upgrade cycle for my computers. On the first year usually upgrade something minor (video card, hard drive, etc.) On the second year the system is, typically, up for a major overhaul with the old parts being sent to my parents. With respect to cases, I do not really follow any specific path - as long as I can fit my components inside with adequate ventilation the case stays in rotation. I recently went, however, through a couple of cases. Had some cheapie from earlier for ages, upgraded to Antec 900. That one did not survive for long though - I needed two video cards and a couple of RAID arrays. Bought Thermaltalke HAF X. That one will survive for several years.