Graphic Card Issues

The other day my computer started displaying weird colors while I was playing a game. It then flickered for a bit before I got a message saying that my display driver stopped responding and had recovered. This pattern continued for a bit before my computer crashed and gave me a BSOD (STOP 0x00000116, nvlddmkm.sys). When I booted it back up again, it would run for a little while before giving out yet again. I tried installing new drivers, old drivers, beta drivers, but nothing worked.

For whatever reason, I decided this meant my GPU was overheating (though it's not OC'd and has plenty of ventilation) and left my computer off for a few hours. When I turned it back on, everything ran fine for the rest of the evening. The problem came back next day with the added bonus of vertical lines running down the BIOS splash screen and the BSOD. Even worse, I could only boot in safe mode.

So I ordered a new card and just used my computer with the graphics card disabled (with my monitor still running through the card). This worked until earlier today when my screen froze completely. When I rebooted, my monitor wasn't getting a signal (not even the BIOS screen). Conveniently, my new card also came in today, however, my monitor still isn't getting a signal. And that's where I stand.

My old card was a GeForce 8800 GTS, the new one is a Radeon HD 4870. I've read that that card uses a lot of power, is my 520W PSU enough?

The 4870 also requires an extra 6-pin PCI-e connector, which I don't have at the moment. So instead, I have the 2nd 6-pin coming from the included 2x 4-pin to 6-pin Y adapter. Additionally, both 4-pin ends of the adapter are connected to the same cable. I feel like this isn't the proper setup. Is this why the new card isn't working? I haven't fried anything, have I?

Lastly, am I barking up the wrong tree, is this a motherboard problem? Or possibly something I haven't thought of?

Sorry for the length, just wanted to make sure I didn't leave anything out. Let me know if there's anything more I need to add. I'm a little lost here.