Day of Defeat system requirements state that you will need at least 4 GB of RAM. To play Day of Defeat you will need a minimum CPU equivalent to an Intel Core 2 Duo E8400. The cheapest graphics card you can play it on is an NVIDIA GeForce 510.
Day of Defeat will run on PC system with and upwards. Additionally it has Mac and Linux versions.
Show Day of Defeat system requirements on your site