Look for a system with AMD's Phenom II series or Intel's i5-750 / i3-530. Post some PCs (full specifications too please, including your planned/current monitor resolution) with those CPUs according to your budget and we can help ya out.
I wouldn't go with anything in the core i3 or lower core i5(600s or lower). They're only dual cores... AMD's Phenom II 955 C3 revision easily beats them on price/performance($130 for lowest i3/$160 for 955)
The Phenom II 955 would be best price/performance but the lowest performer by a small amount.
The core i5-750($200) would be a good choice if you don't plan on crossfire or sli.
Core i7 930($300) if you ever wanted to use dual video cards.