the P4 might cause it to slow down and the ram will. i never saw any PC with DDR4 ram, the 7800GTX will play any game. you can put all the settings on high if u want.
There is nothing called ATI Geforse 7800gtx HD It's called: NVIDIA GeForce 7800GTX made from NVIDIDA. Ati and Nvdia are competiteros
Yes you can use x16 PCI Express video cards on your D915GAV motherboard. I've the same motherboard & I'm using Nvidia GeForce 7300GT PCI Express. You can install all types of latest PCI Expresses on it. Here are the names of some great PCI Express graphics cards of Nvidia: Nvidia Geforce 9800GTX Nvidia Geforce 8800GTX Nvidia Geforce 7800GTX These PCI Express graphics Cards are great for playing games! I suggest you to buy Nvidia Geforce 8800GTX Nvidia Geforce 9800GTX is the greatest PCI Express but it is very costly. If your budget is limited then you can buy Nvidia Geforce 7800GTX Please dont try Nvidia Geforce 6 series PCI Express: it is very old. It supports DDR1 type RAM not DDR2 type RAM. Best of luck.
The best nvidia card currently available is the 7800gtx. However by the end of the month the 7900gtx should be out.