Well, unless you're going to use the fancy Z68 features you might as well go P67.
Get 8gb; it can't hurt. Tom's thinks it's a good idea because it allows your GPU to steal more system RAM for its own use.
What are those old parts?
P67 looks good but I'm thinking of adding an SSD in the future and I'm interested in the Z68's SRT. Haven't yet understood what SRT does but I think it boosts HDD performance. Besides I read somewhere that Ivybridge will be compatible with current P67/Z68 boards so why not choose the more "advanced" model.
About the memory, if the budget allows, I'll pick these instead.
All the old parts that I'll be using are only about 1-2 years old and are still running perfectly. PSU is Seasonic, HDD is WDC and Case is a CM690 with 3 120mm R4 fans.
"Besides I read somewhere that Ivybridge will be compatible with current P67/Z68 boards"
Thats a new one on me, and i highly doubt it as Ivybridge will be an entirely different socket type again, in typical Intel fashion, forcing us to not just buy a new CPU but also a new Intel made or licensed Motherboard.