Yes, I think you should go dual core....it's starting to become the standard. The X2 series are good dual cores. Also I think you meant a 350W power supply. You should go for higher if you're looking at R600 and G80. At least 600W+. If you want those graphic cards, get a mid-range card to hold you for now, if you can't wait, like the 7600GT.
3770K | ASUS STRIX GTX 1080 | DDR3-1600 16GB | CORSAIR 650D | CORSAIR HX 850W | CORSAIR H100 | SOUND BLASTER Z | ASROCK EXTREME4 GEN3