The GPU is a little bit harder to overclock higher that 315 on most 4200 cards. It all depends on the GPU version, A2 vs A3, at time of release (know from Articles read). I even think some watercooled 4600's didn't get much higher than 325. Also you don't get as big as a payback as overclocking the memory for higher resolutions. The key to any overclocking is to make sure your case has good, not fair, cooling. If you have good case cooling you should be able to get some good overclocking. The extremes would required more, but wouldn't be worth it on a 4200 (watercooling, etc.).
-JC