When the phenom II's came out i thought they would be significantly cheaper then the i7's, even though motherboards are generally cheaper the cpu's are not, i remember one of my friends knew a guy who worked for AMD and he was telling my friend that intel's i7's are good but the new phenom II's will be better. Although this was only about a month ago so maybe he is referring to the next revision of AMD chips.
Well you have to go ddr3 for the i7 but if you were to go amd, it would be wiser to get an x4 940 and ddr2 ram. But your rig is much better than a 940 system.