HD5670 is indeed a good chioce but if you have good PSU ( from brands like corsiar, CM, seasonic, gigabyte, tagan etc. ) you can buy a 9800GT 512MB edition card around 5.5k - though it's old it's still packs a powerful punch.
BTW, do post your system details like mobo mdel, cpu, amount of ram and PSU brand and wattage while posting in the future.