Using the stock 435MT Dell case, I was able to fit in a Sapphire Radeon HD 5770 with no issues. Provided I did upgrade to a powersupply that provided higher watts.
I know have the MSI GTX570 Twin Frozr II. The card fit in fine into the old case, but it was seriosouly overheating since the Dell case has bad airflow. I upgraded to a RoseWill Challenger (40$ on newegg) case, and now the temps are super cool. I'm able to play BF3/Skyrim/CS:GO on Ultra settings with the current GTX570 setup. Before, I was able to get med-high settings on 5770