The Core 2 Duo E6550 (65 nm) was released in Q3'07 and the Pentium Dual Core E6600 (45 nm) was released in Q1'10. Don't confuse the Pentium Dual Core E6600 with the Core 2 Duo E6600. A Core 2 Duo E6550 will bottleneck a 6850/6870/GTX 560 Ti. So will a Pentium Dual Core E6600, but to a lesser extent. For a balanced system, you should get a more powerful CPU.
If you check the benchmarks, a Pentium Dual Core E6600 gets 2,103 on the CPU Mark score while an E6600 gets 1,508 and the E6550 gets 1,456. I may be wrong, but 2,103 probably is better than 1,456. http://www.anandtech.com/bench/Product/61?vs=94
It's a comparison of the Pentium E6300 (they didn't test teh fater Pentium E6600) and the Core 2 Duo E6550. If the Pentium E6300 usually is faster, then I would presume that the Pentium E6600 is faster.