Core 2 Duo's kick the ever living snot out of pentiums. For example. I used to have a Pentium D 820 that I'd overclocked to 4GHz. The very moment I installed a Core 2 Duo e7400 at stock speeds my framerates in game jumped across the board. In Far Cry 2 alone it went from appx 25fps to well over 45. This was with an overclocked 4850 I had at the time. So, I reiterate. CORE 2 DUO
The E6750 is faster, but without knowing the price difference and your ability to pay it I have no clue if its worth it to you. If you don't overclock and need/want a fast chip the 6750 is the chip for you.
Yes, as does the faster FSB. Thats what makes up for the slower CPU speed. As Timop said at stock these should be very close together. Seeing as the 6500 has a higher multiplier and runs on the 45nm process it should overclock better.