You trying to update the graphics card? What type of graphics card you have? If this isn't known yet just open run and type, "dxdiag."  Once this pops up click the display tab. Under name will name your grpahics card. Once you know this tell me.

Sorry I replied incorrectly just ignore that. 

Windows XP reached end of life in April last year is insecure and should no longer be used.

This system should run Windows 7/8.1/10 64 Bit without any problem.

System drivers are here:

http://dellwindowsreinstallationguide.com/driver-sets/dimension-desktops/dimension-e520-windows-7-64-bit/

I advise installing the Windows 10 Technical Preview and evaluating it until its released when you can take advantage of a promotional price:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/a-clean-install-of-windows-10/

Installation of Windows XP is not recommended however if you insist on installing such an obsolete OS, you should slipstream SP3 and SATA drivers to your installation media:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/obtaining-the-latest-service-packsbuilds-of-internet-explorer/windows-xp-service-pack-3-standalone-updates/slipstreaming-service-pack-3-windows-media-player-11-and-sata-drivers-into-your-windows-xp-reinstallation-media/

With XP, the 7B error means SATA Operation is set incorrectly in BIOS setup.

Since XP doesn't natively install SATA drivers, you have to set SATA Operation (aka: SATA Mode) to RAID AutoDetect/ATA in BIOS, and then install the SATA driver from a floppy diskette by pressing F6 when prompted near the beginning of the XP installation.

If you don't have a floppy drive in this system, you can skip the SATA drivers and just leave BIOS set to RAID AutoDetect/ATA after XP is installed.