Your Dell may have a board with a x58 chipset, but it is a long, long way from a retail board. This is something people don't understand when they buy prebuilds.
To keep cost low, Dell orders a chopped down version of the board with the bare minimum to run an i7 processor, and a single dedicated GPU. Your board only has 1 PCIe slot, and includes no support for SLI or Crossfire. Your system also comes with fairly underpowered 380 watt PSU. One review put the total wattage pull when the system is using all cores of the i7 at 360watts. So your PSU is not going to hold up to any type of graphic upgrade, even a better single card is going to cause you big problems. If your system suddenly refuses to boot one of these days, the PSU is the weak link in this system. (Unless you upgraded it when you bought it)
But, what you do have is a pretty fast i7 that should play most modern games at upper mid range settings, and the price you paid for the system is really quite modest for an i7 based rig. But you are going to have to be happy with it the way it is I am afraid.