Understanding RAM characteristics

Question

Hey Ginko, I was hoping you could help me out with this one:
What's the diff between say, 4gb of 667 or 800Mhz ram and 2gb of 1066 or 1200? Is less Ram at high speed a better option? I guess it goes without saying that 4gb at 1200Mhz would be ideal, but I dont think I can afford that.
Mobo is ASUS p5q se/r