|I posted this link on your first thread but u didn't bother to check it, so i'm posting it again|
From the link, look at description u can see all three form factors of HP Compaq dc7800, yours is Convertible Minitower, scroll down to Memory and u can see supported memory with their speed.
Now what PCI is it? 2.0 or 2.1 ?
The Intel chipset Q35 was released back in June 2007 and PCIe 2.0 was released back in January 2007 so it must be PCIe 2.0. Don't worry the HD 6570 is PCIe 2.1 card and it works even with older PCIe 1.0a slot.
The other is in use for video editing.
I think u talking about your other system(the one with 8800GT and ASUS board)
Video editing rely heavily on CPU, u need a high end CPU for the that task. Also the old 8800GT isn't enough for video editing
You know the real meaning of peace only if you have been through the war.