What will you be predominantly using the computer for? That will ultimately indicate what the hardware requirements are!
A 32bit operating system (OS) will only realize 3Gbs of RAM. Tests on this Site have shown that for most applications between 3Gbs and 6Gbs of RAM on a 64bit OS will suffice. RAM is cheap so most OEMs will through RAM at you as a bribe.
If you indicate you general use I may be able to offer more advice! Cheers!