Start off slow with your overclock, take it up nice and easy and stress the board a bit before going to the next step, 5MHZ increments or so. Usually, you will start to see artifacts on the screen long before running in to stability issues with the Ti boards, so kick it up a bit at a time until you hit your limit. Looping 3DMark2K1(1600x1200x32bit 4x AA and 8x AF, a 'slideshow' that stresses the hell out of the board) is my favorite method for testing out the vid card when testing an OC, although you may find something more to your liking
The only difference between the Ti4200, 4400 and 4600 is the speed that they run at, if you have your Ti4200 running at Ti4400 speeds then you essentially have a Ti4400(you may be able to hit Ti4600 speeds, but don't count on that).