Just to expand on what Denther wrote
Look for...
i5 2500K
Good cooler for Overclocking
Motherboard with a P67 or Z68 chipset. Preferably ATX from Gigabyte, ASUS, MSI, ASRock. Socket 1155.
4 or 8gb of DDR3 1600 at least CAS 9 if not lower
GPU of your choice. Look for AMD 6850, 6870, 6950, 6970, OR nVidia GTX 560TI, 570, 580.
A GOOD PSU that meets the requirements for your GPU. XFX/Corsair/Seasonic/PC Power & Cooling/Antec have some good ones
Case with good airflow
CD/DVD drive
HDD- STAT II or III 7200RPM From Segate Western digital or Samsung
Windows 7 Home Premium
Mouse/keyboard
Monitor- LCD or LED 1920x1080 at least 21.5"
I probably forgot something. Good Luck