well, think you should start at about 325/675 since the stock clocks are 300/650 Just increase the core and mem about 5-10 mhz at a time and then run some benchmark loops to make sure it's stable.
I think just running the game tests will not show you all the problems. Most of my Locks from too high an overclock come under one of the Specific tests after the Games tests. Really want to test it, Run Codecreatures Benchmark for a few loops.
Too high a Core clock will usually lock the program and too high a memory clock will usually start to show pixelation and flashing textures, when you reach that point back off a few clicks and you should be good to go. I'm not sure about core speeds but I see Memory speeds up around 720-725Mhz for the 4600. My Ti4400 would do about 660, and my 4200 will do about 620Mhz. Good Luck.