I need a graphics card

Okay so i finally got nearly all of my new computer:

AMD athalon 64 3700+
200gb SATA harddrive
2x 512mb kingston ram 400ddr
Asus AN8 duluxe motherboard (SLI, pci express 16x, dual ram slot for ddr400)
A good case with 450watt PSU

Now i basically need a graphics card, im not bothered about the sound card at the moment. My budget is £140. Im obviously not going to be able to utilise the SLI feature as it would be to expensive. So im probably going to go for an ATI card. Im not sure which one, id like to get an x800xt or an x850xt i think. Any suggestions on a card or where to get it from ?