Ok your confused. Ram isn't rated by ghz, it's rated by gigabyte (gb).
Which models are your referring to specifically?
If your asking what's better, a Pentium dual core 3ghz or a i3 dual core 2.64ghz, it depends on what models specifically and what you use the computer for.
Are we talking about a sandy bridge pentium?
i3 has hyperthreading, pentium does not have hyperthreading. If it's a sandy bridge pentium, then the higher clock speed would be faster in normal tasks than the slower i3. If you use programs that utilize hyperthreading, then the i3 would be the way to go.