HP Pavilion 500-164 | What GPU should i get? |

I am not the smartest when it comes to Upgrading a Computer. Recently i got my self a HP Pavilion 500 - 194 

HP Pavilion 500-164

So I wanted to upgrade my G-Card without upgrading my Power Supply and Mother Board 

I Play games like Garry's Mod and Team fortress 2 , which i can max out but as Gaming progress my PC cant handle it.

I was wondering if anyone would recommend any GPU's that would work with my Computer.It would be also grateful if anyone would tell me problems i will get when inserting my GPU in (if there is any) . Thank you and enjoy the rest of your day