Is a 600w PSU Enough??

Hi - You need to post your sys specs in order to determine your
power needs.
Tom

Specifically you need to provide details of the PSU.
Most stuff outside of graphics/CPU is reasonably low power and doesn't draw much.
The deciding factor will likely be how many watts the 12v rail(s) are capable of putting out, and whether or not a multiple rail system gets split awkwardly.

System Specs:

CPU: AMD FX-8320 @ 4.2 GHZ
GPU: MSi GTX 660ti Power Edition
Mobo: Gigabyte GA-990FXA-UD3
SSD: OCZ Solid 3 60gb
SSD: Kingston Hyperx 3k 240gb
HDD: WD Velociraptor 500gb
RAM: 16gb Corsair Vengeance
PSU: OCZ Modxtreme 600w
Corsair H60

You should be ok, but it's not especially comfortable. The power draw will be reasonably low, but because you have 3 high draw components and two rails, you'll probably end up with one rail quite heavily burdened. 150W+150W+130W is getting pretty close to your 500W 12V cap, and that's assuming you are accurate in how much power your CPU is drawing.
Also, that PSU only has two PCIe connectors, and each of your graphics cards would need two. You can use adaptors but it's not a very good idea.
If you are confident your CPU is only drawing 130W, then in theory you should just about manage, but I think you'll realistically have to look at a new PSU to be comfortable.