Well, you didn't give what model your HP pavillion model is. But I checked a few model and they all using the same power supply. It only has 180W pure power supply. So in order to upgrade your pc, you have 2 choices:
1. Use original psu, but gpu were limited. Its cheap though
ZOTAC GeForce GTX 750 Ti ZT-70605-10M 2GB 128-Bit GDDR5 PCI Express 3.0 HDCP Ready Plug-in Card Video Card
$100
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500349
2. Change your power supply and get a much better gpu. Well in this case, you forgot to mention your budget. But maximizing your 4 core amd cpu, i will recomended to get a R9-380 or a 280x. 380 would be more likely as your cpu might barely optimized the gpu.
XFX Radeon R9 380 R9-380P-2255 2GB 256-Bit GDDR5 PCI Express 3.0 CrossFireX Support Double Dissipation XXX OC Video Card
$190 ($170 if you can rebate)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150732
For power supply, here is the best value
CORSAIR CX series CX600 600W ATX12V v2.3 80 PLUS BRONZE Certified Active PFC Power Supply
$50 ($30 if you can claim rebate)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028
Total $240 ($200 if can claim rebate) but it will certainly transform your pc ability to run games.
Please pick as best answer if helps