Just installed new GPU, worked fine for several days, now I can't play any games

Hardware Hub GPU
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The title basically sums up what's wrong. I recently went from a GTX 550ti to a R9 280. For the first few days everything worked fine, got a nice boost to performance, despite the 280 being held back by my CPU and Motherboard having not been upgraded yet. Now after I went and deleted the drivers from the 550ti nothing works. I tried playing battleblock theater and it told me it ran out of memory, and sniper elite 3 tells me I need a direct x 11 compatible graphics card (which I do) and basically all my other games just crash, excluding games bought from GOG.com. I've tried all the obvious things, restarting the computer, making sure the newest version of direct x was installed, and updating drivers. Unfortunately I still can't figure it out, any help is much appreciated.

Other PC specs: OS - Windows 7 64bit (fully updated) Mobo - Biostar T Series TA870U3 CPU - AMD Phenom II X2 555 Ram - Kingston Hyperx Blu 8 gigabytes HDD - Seagate Barracuda 2TB SSD - PNY XLR8 120GB (Yes I realize my CPU and motherboard are kinda crap, I'm working on it)

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Im far from an expert on this, but it does sound like a driver problem. I found two threads that might be helpful for you

(This one looks like you're exact same problem) http://www.tomshardware.com/answers/id-1907804/radeon-7870-suddenly-stops-working-uninstalling-nvidia-drivers.html  

http://forums.whirlpool.net.au/archive/1999611

It seems like there may be traces of you're Nvidia drivers.

Here is a link for driver sweeper if you want to give it a shot, http://www.guru3d.com/content-page/guru3d-driver-sweeper.html I'd say boot in safe mode and uninstall all current video drivers and then reboot andinstall the latest from AMD's website

I hope you can get this sorted out.

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Worked flawlessly, Thanks so much. That first story was pretty much exactly what happened.