best GPU

[pjn]

I have just been given this PC (shown below) for free and so now need a graphics card, hard drive and OS to get it started however i am not sure what GPU to get:

Spoiler

- XFX x58i Motherboard
- i7 920 (1st Generation i7 processor)
- Quad Core, with HyperThreading
- 2x Corsair 4GB DDR3 1600MHz RAM (Total of 8GB of RAM)
- OCZ 700W Power Supply Unit
- Aerocool AeroEngine-Plus Case

I would like the GPU to be compatible with this over clocked bundle (shown below) as i may upgrade if the pc with the GPU isn't good enough or powerful enough as i know the bundle is better:

Spoiler

http://www.overclockers.co.uk/showproduct.php?prodid=BU-000-OG

 

[pjn]

budget is less specific as i only need a GPU so far but the sort of games i'll be running is borderlands 2, space engineers and DayZ