CNET shows thes specs for an OptiPlex GX520 with a Celeron D 326 2.53 GHz, 512 MB DDR2 RAM and an 80 GB SATA 1 hard drive. The puny 230 watt PSU will be another limitation on any video card you might want to add.
Even assuming you have a version with a stronger CPU, a bigger hard drive and as much RAM as the machine will take, trying to upgrade this machine for HD video would probably cost you more than buying another machine, and no matter how much you try to upgrade this one, it can only disappoint you.