Try Iobit Driver Booster. The free version will find the drivers you need. (When you install it you might want to uncheck the box that offers to install Advanced SystemCare Ultimate BTW) Hope that helps. And to avoid all the nag screens that will surely plague you uninstall the software after you're done updating the drivers.
Also, have you tried going here: http://www.dell.com/support/drivers/us/en/19/Product/optiplex-gx280
Lists the available Dell drivers for your system, though they are all dated as 2005.