Hello, if you were told by an expert tha you need a video card, here what you need:
1: You do not have a slot card type video card in your system( Dimension E521) your system has on board video, which you can not remove from the system but disable it.
2: Your system has PCIe x16 slot that can be used for new video card.
3: Link below will give you a list of PCIe x16 video cards, you can pick and choose according to your needs. Do you need DVI (digital connection, does your monitor has a white port in the back or just the blue one?), or regular VGA (blue port) is enough, do you need HDMI etc...
bottom line is any of those video card will work.
4: I am assuming you have never done this before so before you open up computer and start taking things apart go to the link below and download service manual for Dell Dimension E521 ( I selected the model for you so you don't have to look for it :-) )
5: In the service manual link there are several Sections like:
Before You Begin
About Your Computer
Removing and Installing Parts
click on each one and read about it first to familiarize your self with the system.
6: Once you feel confident enough remove the power cable from from computer.
-> open computer cover
-> follow instructions in service manual under
->Removing and Installing Parts
->PCI Express Cards
-> Installing PCI express card.
-> Close computer cover
-> plug monitor cable to new card
-> power on the computer
-> if Picture quality is bad that means card was not properly recoginzed, install drivers from the CD that came with the video card.
I hope this helps, if you get stuck at any step or not clear feel free to post back.