Have $500-700 to spend on upgrade for gaming PC.

Hey I was looking to upgrade my PC to be able to run basically every game on highest settings. I don't really have a preference between AMD or Nvidia for my GPU. Basically I'm asking if this was your PC, and you had about $700 to upgrade this rig, what would you upgrade in order to achieve the aforementioned goal?

Mainboard : ASRock Z68 Extreme3 Gen3
Chipset : Intel Z68
Processor : Intel Core i5 2500K @ 3300 MHz (overclocked on water to 4500MHz)
Physical Memory : 8192 MB (2 x 4096 DDR3-SDRAM )
Video Card : NVIDIA GeForce GTX 570
Hard Disk : ATA WDC WD20EARX-00P SCSI Disk Device (2000GB)
Hard Disk : Intel Raid 0 Volume SCSI Disk Device (240GB)
Monitor Type : Samsung SyncMaster - 27 inches
Monitor Type : Gateway HD2200 - 22 inches
Network Card : Realtek Semiconductor RTL8168/8111 PCIe Gigabit Ethernet Adapter
Network Card : Realtek Semiconductor RTL8188CE 802.11b/g/n WiFi Adapter
Operating System : Windows 7 Professional Professional 6.01.7601 Service Pack 1 (64-bit)
DirectX : Version 11.00
Windows Performance Index : 6.8 on 7.9

I appreciate ya'lls help.
Sean.