I decided to upgrade my htpc(pentium g4400, gt 710, 8gb RAM and a 430w power supply) to something that might be a little more capable. I found an hd 7770(edit, hd 7790) for £15 which is a nice price for a small upgrade. However, now, my PC won't boot. Either with the gt 710 or the hd 7770(edit, hd 7790), nothing happens. What's wrong?