You cant use PCI-E video cards with a 520 because the X16 slot is removed as are 2 of the ram slots.

My dell has a pci slot inside there is a thing that you put in the slot at the bottom it looks similar to a pci slot but that has a adapter thingy which has 2 PCI slots 

You are referring to a Riser card.

There are no reasonably priced 32 bit 33mhz vaccuum tube PCI video cards that are of any use. Nobody makes PCI 2.2 cards anymore.  All cards are now PCI-E

Older PCI 2.0 cards are 5v only and do not work at all.

Universal PCI 2.2 card

A better solution is to just give this system to charity an get another one. This thing would be bottlenecked if it could use a modern GPU my old 780 is not doing too well with a GTX1050ti since it will occasionally get bottlenecked on a Core2 Quad Q9650. Imagine what would happen to a Pentium 4 (*it would not live to see another day)

actually if it is possible to update the pc gx 520.

-the maximum number of ram is 4gb, if it doesn't work for you, try 3gb

-I'm still inventing which is the best graphics card.

-the best processor for this machine is: an intel pentium 4 dual core at 2.8 gigahertz, or an intel celeron D at 2.6 gigahertz.

What form factor/size is your GX520 and what are some the games you want to play?  Games system requirements can also be Googled.