you can overclock as much as you wish, as long as you maintain system and video stability... just clock up by a few mhz each time to figure-out what your top core and memory speeds are... you can use the manufacturer's OCing utility or something like RivaTuner
<b>people are only idiots when they don't realize - when they do it just gets funnier, like a dog chasing its own tail, or like george bush's public address(es)</b>
I have a LeadTek model of that card, and mine is flash written to 275 GPU and 510 memory. Leadtek makes exceptionally good cooling solutions.
275 is exactly 10% up from the default speed of the core, and the same level as the 4400, and 510MHz is about the memory speed of the 64MB version of the 4200 (that speed is 514)
General rule of thumb, you don't want to overclock the core of anything (be it a CPU or GPU) much more than 10% Memory overclocks a little better. Unfortunately the 4200 models with 128 VRAM use lesser quality RAM than the 64 meg versions (this is to keep the price lower). I believe the access time for the 128MB version is 4ns (nanoseconds). Regardless of heat, 4ns can only go so high without becoming unstable, that is why I don't go above 510 with mine.