Should I upgrade my pc?

So i'm going to buy a new graphics card (7870) and a new psu (xfx 550w), and i thought that maybe another component in my pc would bottleneck it, seeing as i was on a £580 budget when i bought this, including monitor, mouse, keyboard and speakers.

My specs are :
Motherboard : Gigabyte GA-MA770T-UD3
CPU : AMD Phenom II X4 955 @ 3.20GHZ
RAM : Kingston 4GB DDR3 1333Mhz
GPU : Saphire ATI Radeon HD 6670 ( upgrading to xfx 7870 core edition )
PSU : TOQO 480W Silent ( I don't know anything about the company or the psu) ( Upgrading to xfx 550W Pr)