Ok, I just got a new video card today (ATI Radeon x700 Pro)

I installed it into the PC and then when I go to boot up my PC, the PC boots but no display comes on the monitor.

I have 350W and it only need 300W. I think I might need to update my drivers or BIOS or something. My motherboard manufacturer is at www.viaarena.com. And It said I should obtain and install the latest AGP drivers and installing the wrong AGP Motherboard/Chipset drivers may prevent me from successfully launching Windows.

I went to http://www.viaarena.com/default.asp...ID=1&CatID=1070 and downloaded the 04 January 2006 Version - 5.07A. Installed it etc. and it still doesn't work. I disabled my integrated card also. I've never installed anything on my PC before so I don't really know what to do.

On another page in the manual it says for No display at boot to Make certain the motherboard chipset drivers are installed and/or I may need to update my system BIOS. I did a update on my BIOS but it didn't help.

I have a HP Pavilion a420n, The motherboard A7V8X-LA.

I just want to install my video card! I didn't think it would be this much of a hassle.

That computer came with 256 MB or memory. The memory will have to be upgraded to at least 512 MB if it has not been. Also, you may have to install Windows XP Service Pack 2 if that has not been done, as that machine came with an early version of Service Pack 1. There are a very large number of downloads since February 2004 for that machine. You may need to upgrade th BIOS and run many of the other downloads from the HP web site. I would install all of them.
Finally, check the Device Manager in Hardware in System at the Control Panel to look for devices flagged with a yellow or red mark... after you have downloaded everything.
Do not discount the idea that the video card itself is bad. That happens sometimes.