It all depends on what you want out of the system if you intend not to buy new parts but second hand ones sold on e-bay ect.
Or what you intend to use the system for mostly valhallarising.
The older the parts the cheaper they will be to buy.
But the less you will be able to do with them if your intention is to run the build with modern software, or mainly games.
I deal is to opt for at least a quad core cpu.
And a second hand motherboard with a good chip set on it.
One with lot`s of expansion possibility, and at least four memory or ram slots provided on the motherboard.
As for memory that changes pretty rapidly in the type and standard used.
So the type of memory the board can take and how much it supports should be thought about...