A little correction here. If your choice is HD 5670 then get the 512 MB GDDR5 version which is available at a less than 4K price point. The reason being that is there is no real world performance difference between the 1 GB and the 512 MB model. Check online store Theitwares where the Sapphire HD 5670 512 MB GDDR5 is available at 3.9K.
But if you can extend your budget a little then HD 6670 1 GB GDDR5 is the best option. Get the MSI or Sapphire version..available around 5.2K to 5.4K.