Ok, i recently bought a E5700, 3.0GHZ, 800bus, 2MB cache Dual-Core CPU. It satisfies me mostly, but a few CPU-heavy games like GTA IV, The Saboteur etc are lagging on high settings. So, i decided to try its performance after overclocking. My motherboard allows me to overclock it upto 3.75GHZ, with 1000bus. The performance increase is easily noticeable. (about 10+ FPS on the mentioned games, and much more on other games), but the problem is the increase in temperatures on full load.
Your temps might be OK for use because you normally won't be running at full load constantly, but you could also monitor your temps while gaming and see how they compare. (Assuming you are running something like prime95 to obtain your "full load" temps.)
I normally would not attempt to overclock without a good aftermarket cooler. With a good cooler your overclocking temps might be lower than what your temps are with a stock cooler at stock speeds.
The temps recorded by the cores are not 100% accurate, and there might be some variation in how your cooler was installed. The OS will share the load between the cores equally. You can probably see this if you watch task manager while gaming, or even a program like coretemp. You'll notice that the OS switches the load back and forth between the 2 cores, not running either one constantly.
the max operating temps that intel considers to be safe is 74.1C
seems like you are as hot as you can get without doing severe damage. move your case to a cooler area, get better fans, get a better heat sink, or go sit in a walk in cooler.