Well Gtx 560 Non Ti is on par with 6870.
So u will do game. I linked u a benchmark where you can see game running with Hd 6870.
Your PSU its not even a brand :S Im worried about it.http://www.hardwaresecrets.com/article/Antec-VP450-Power-Supply-Review/1487http://us.ncix.com/products/?sku=62331&vpn=VP450&manufacture=Antec
Consider getting this and it will suffice you. You wouldnt have to worry about it
IMO you should really buy this PSU. That HP psu its scaring