You're not running prime in the background are you?
Nope.
You're not running prime in the background are you?
I'd strongly suspect one of the RAM modules.
Try swapping out pairs if you have 4 or more.

Of course the GPU's can come out. Towards the back of the PCI-E slot is a little catch you normally have to push it sideways which releases the card and it will then lift straight up.
Well, I just checked dxdiag and it says it only recognises 8gb...... something isn't right :/