If an Athlon II X4 bottlenecked your GPU, running a 2nd one in crossfire would only further create a larger bottleneck, not fix it.
You didn't bother to mention your resolution, but a 5670 is honestly weak enough that even low res gaming the Athlon II should keep up just fine. If your gaming at 1680x1050+ than I can say for sure the 5670 will actually be your weak link in games.