Desktop Hardware and Upgrade Questions

I bought a HP Pavilion p6-2104ed years ago but wanted to change the GPU. I transported all the hardware into a new case (Coolermaster 280), upgraded the standard PSU into a Seasonic 650 W 80+ Gold, and bought a new GPU (GeForce GTX 960 2G).

The computer works perfectly in the new case with the old GPU (HD Radeon 7550) but does not boot with the nw GPU.

When I start the computer, the first start up screen shows up and a total of three beeps appear before the screen turns black with a little flickering dash in the upper left corner. It is like this: https://www.youtube.com/watch?v=M9F0LWwZD3w

-I did connect the GPU with a PCI cable for the power and its lights and fans turn on.

-There should be enough power to run everything (650W).

-I deinstalled the drivers before installing the new GPU.

-I updated the BIOS via this link: http://support.hp.com/us-en/drivers/selfservice/hp-pavilion-p6-2100-desktop-pc-series/5187028/model/.... (not completely sure if I succeeded tough)

-I doubt that there is a problem with my RAM as the three beeps can indicate because the computer works fine with the old GPU.

What can be the problem and how can I solve it?

-Does it maybe have to do with Legacy / UEFI settings? The MoBo should be compatible (http://support.hp.com/us-en/document/c03235578).

-Is there a problem with the BIOS?

-Does it maybe help to disable the integrated GPU? If yes, how. I cant seem to find the right settings.

-Am I misunderstanding something?

An overview of all me specs (Partly Dutch):

-Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz, 3401 MHz, 4 core('s), 8 logische processor(s)
-Mobo: Foxconn (H-CUPERTINO2-H61-uA TX (Cupertino2))
-BIOS-versie/datum AMI 7.16, 23-3-2012
-Geïnstalleerd fysiek geheugen (RAM) 6,00 GB
-Totaal fysiek geheugen 5,98 GB
-Beschikbaar fysiek geheugen 2,50 GB
-Totaal virtueel geheugen 12,0 GB
-Beschikbaar virtueel geheugen 7,35 GB
-Ruimte voor wisselbestand 5,98 GB
-Functioning GPU: AMD Radeon HD 7500
-Not Functioning GPU: MSI GeForce 960 GTX 2G

Help is much appreciated!