CPU speed is like the RPM's in your car. They don't relate directly to performance, so there is no "Equivalent speed" The core 2 duo would be faster than any single core processor, even a 4GHz pentium 4.
Many factors other than CPU speed will affect performance of a computer. All things being equal, Core 2 Duo is better, but you will also want to look at RAM, HDD speed, Operating system, and Video Card. If you post this information, you will likely get a more useful answer.
Everything else is the same just have a choice of Core 2 duo 1.8ghz or pentium D dual core 2.8ghz
So is the core 2 duo better despite so much less ghz?