As
cdub suggests, you need to use 3rd party sw to allow core & RAM clock mods and 3Dmark is great to verify your limits (but I'd suggest backing off a little more when you find it).
4200-128MB are just as capable of hitting 320/620 as 4200-64MB. 4400=275/550 o/c=300/620, 4600=300/650 o/c=320/720. Doesn't anybody read the other threads? IT COMES DOWN TO THE TYPE OF RAM IMPLIMENTED!
Core on 4200 cards should o/c anywhere from 280 to 320.
4.0ns is standard for ALL 4200 cards and o/c to around 550.
3.6ns is more common on 4200-64 but is also readily found on 4200-128 and o/c to around 620.
You can read the ns rating of your RAM from the RAM chips, 2nd line down last 2 numbers. Fancy HSF designs do VERY little, RAM HS do nothing to enhance o/c. Manu is of little importance, the chipset dictates almost all perf, features and potential.
Any 4200 card is great value for money, even at defaults they beat the GF3TI500 and full ATI Radeon8500, however the 4200-128MB is definitely the better buy, esp long term (6 months+).