You should be able to approach 300mhz core clock with any GF4, as they are all A3 revision cores (GF4 Ti4600 was A3, Ti4200 was released later and revealed its the same rev. core). But yah, as others said, the main factor controlling memory overclocking is simply the rated speed of the RAM modules on the card. I had an Albatron Ti4200 Turbo @ 310/650, could've possibly pushed it further but I set it and forget it.
Chiz