Depend on what you are doing with the PC. In 3D, video rendering and other aplications where you need a powerfull cpu an 6 or 8 core are the kings. For gaming a dual core or a quad core is enough depends on the games what you play. For surfing the web a sigle core is enough.
ask in the system section for info on whole pc. if you're gonna build a new pc, then use this form, open a thread in the new build section. http://www.tomshardware.com/forum/261222-31-build-advic...
cpu choice is mostly personal depending on tasks. imo,
most basic tasks - athlon ii x3 445, sandy bridge pentiums (g6xx, 8xx)
basic tasks with light gaming - core i3 2105, 2125, amd a8, a6
entry level gaming - athlon ii x4, phenom ii x2, x3, x4, core i3
mid range gaming - athlon ii x4, phenom ii x4, x6, core i5
high end gaming/ enthusiast / video editing/ heavy rendering- core i7
server - xeon, opteron.