Just looked up the Radeon 5870, its teraflop(processing power) rating is impressive. Only about 5% lower than a 7950(non boost edition). Anyone looking to upgrade from a 5870 should get a GTX 680 or Radeon 7970; otherwise they will be disapointed.
Anyway the Titan's rating of 4.5 Tflops, makes it only 45% more powerful than a 680 (3.090 Tflops). Still a good improvement thou... if it only uses 235 watts (~20% more than a 680).