I am gong to build a new system using AMD Athlon II X4 630 paired with GIGABYTE GA-MA785GT-UD3H & Kingston PC3-10600 1333MN9 (2Gig X 2 Sticks) .
To What extent can I OC using the stock cooling setup & OCZ Modxstream-Pro 500 Watts -- OCZ500MXSP?
I am not a gamer, may use the system mostly for music, movies & surfing and occasionally for CAD applications.
As long as you have a single graphics card, you can OC to as high as a voltage as you'd like (somewhere in the 1.4V-1.45V range, OC of 3.6ghz or higher), though if you're sticking to the stock HSF, you can probably OC to 3.0-3.2ghz on stock voltage alone. Most of your applications aren't very intensive, so you might find out you don't even need to OC.
I have the same mobo and CPU as you, and I got to 3.5GHz @ 1.55V on stock cooling. It would get up to about 70C after a few mins of Prime95, but overheating wasn't a problem under normal use. I've recently got an aftermarket cooler and it keeps the temps down, but won't allow me to overclock higher.
But anyway, Kokin's right that you probably won't need to OC...