A pair of HD 7970 GPUs would need a serious workload to bottleneck an Ivy Bridge i5. If you are going to overclock then don't worry about it. IMHO overclock the i5-3570k to an effortless 4.5GHz and call it done. Add your second GPU when you see fit.
EDIT: Don't forget an aftermarket CPU cooler if you are going to overclock.
nice to hear that! well what if i dont oc so much i mean i am a newbie about overclocking i havent done any serious oc yet do u think i can face some problemswith stock clock or oc to 4 GHz maybe 4.1