The cheapest 6870 on Newegg is $170. If you can afford that you can afford the 7850 that xXStavrosXx
already recommended, which is also my recommendation and the 7850 will last you a LOT
longer than any of the other 3 as it's the current generation.
Benchmark here Radeon 6870 vs 7850
which shows the 7850 destroying the 6870 while using less wattage at both idle and load (in case you were wondering about your PSU)