4.2GHz is not really big overclock unlike mine at 4.5. I'm using stock coolers and my system Temps are running fine. Partly the reason behind is my GPU Cooler from the MSI Cyclone II's. So in your case, stock coolers are fine if your GPU fan/heatsink can handle it.
Still...just in case you're abit scared of heating, then by all means go ahead and add another cooler.
i just want to know about requirement of cooler for overclocking
to 4.2 ghz
do not oc on stock cooler.
get a cheap but well performing cooler like coolermaster hyper 212 evo.
@UltimateDeep: i think OP is talking about cpu overclocking, not gpu overclocking (topic says intel i5).