you can squeeze larger gpus into medium towers, but for optimal airflow and cooling you will want to use a full tower. especially for a system with components like yours, cooling will be important.
if you are set on a medium case, check out the Antec P180, or the Antec 900.
also, you might want consider a better PSU than the one you suggested, the GTX285 needs 46A on the 12v rail, the one you chose has dual 18A rails, which would only give you 36A maximum on the 12v rail. Take a look at the PC&Power and cooling 750w PSU