take a look at the picture. your colors may vary but the slot will have 1 short segment about 1/2 an inch and then a longer segment.
The recommend power supply is 400 watts and requires a 6 pin pci-e connector. You will most likely have to upgrade your power supply as most dells will come with a 300 watt PSU. Open you pc and check for a 6pin connector (pretty easy to spot as it's unique) comming from the PSU. IF you don't have one then i would upgrade the PSU.