P43 or P45 chipset????

That really depends on how you are going to be using your system.
If you are not going to be overclocking and using only a single GPU, you will see no difference between the two platforms.

The differences between the P43 and P45 are minor.
They are both PCIe 2.0 and have the same feature sets.
The P43, however is binned lower and looses support for 8x 8x CrossFire.
As a lower binned part, do not expect the motherboard manufacturers to put the amount of work into a quality Mb as they would with a P45 Mb.
The result being lower probable overclocks.