I have a Antec 520W psu and my friend is going to give me a Galaxy Radeon 6870.
On the website for AMD it said i jsut need a 500+ psu and i'm wondering if i'll need a new power supply before installing this card.
The video card should work just fine with that power supply , it has 40a on the +12v rail and that's enough to power that card. A GTX 580 requires 39a on the +12v rail so the 6870 will be fine and actually you'll have more than enough amps.
that 520 will be fine as inzone stated; it has two 6 pin power connectors needed.