Dell motherboards are quirky. Such as a friend of mine got himself a brand new X1600 video card PCI-E for his Dell. Got a new PSU to handle it to, but the motherboard wouldn't take it. It had a PCI E slot, the right power requirements, but just wouldn't accept the card. So he had to take everything back and got a x800 instead which by some miracle worked on his Dell.
Here is a few words of advice for upgrading. First, don't try to upgrade a Dell. Its more of a pain than it is worth. Just deal with the fact you made a bad purchase and move on. Secondly, get a computer that has room for upgrades. PCI E slot is required for new video cards. Lastly, know your system. Know the model number. Know its internal layout. Know what it can and cannot do.
Without this knowledge you will always make poor purchasing decisions when it comes to your PC.