No real major differences between Z68 and P67 other than Z68 can use the integrated graphics in the CPU and can do SSD caching. Supposedly both P67 and Z68 will be able to support Ivy Bridge but we won't know either can for certain until Ivy Bridge is released.
Also at the moment Pcie 3.0 does nothing but potentially support next gen GPUs, however until they are released we won't know if it will hold any advantage over running the cards in pcie 2.0.
EDIT: To make an actual recommendation if you'd like to stick with P67 go with the MSI or Asrock I listed in that section. If I had to just pick for you I'd say go ASRock Z68 PRO3 for single GPU, ASRock Z68 PRO3 GEN3 if you care about single GPU & pcie 3.0 or ASRock Z68 Extreme3 Gen3 if you care about possibility for multi GPU later.