How do I know if my power supply is good enough?
My motherboard has 4 PCI-E 2.0 16x slots.
And here's my computer case:
-snip-
Open up your case and look on the side of the PSU. It should have the wattage, brand and model.
I will need to know the wattage and model so I can tell whether or not it will support it.
Are you sure that your mobo has 4 PCI-E slots? What is your motherboard model? It is rare unless you bought a high end mobo.
It looks like your case is a micro ATX. To be sure whether or not it would fit a 6850, I'd definitely measure the length parallel to the top PCI-E lane, from the rear PCI brackets to the nearest thing opposite.