Are the Intel Pentium D and Intel Core 2 Duo processors the same?
No. Pentium D is basically two P4 Netbursts on the same silicon. Core 2 Duos are newer, faster, and more efficient.
Even though they share the same socket- Socket 775- and may run on the same motherboards, they are not the same.
The Pentium D uses 2 Netburst architecture CPU cores on separate dies (two chips, one package), each with a small cache, sharing a single "quad pumped" Front Side Bus of 533 or 800 MT/s throughput (4 x 100MHz x 2=Double Data Rate) for desktop processors.
First generation Smithfield Pentium D's used a 90nM process and have very high power consumption. Each core has a small 1MB Level 2 cache.
Second generation Pressler Pentium D's used a 65nM process and have a somewhat high power consumption. Each core has a moderate 2MB Level 2 cache.
All Pentium D's are 64 bit and some Pressler's support VT for advanced virtualization. Extreme Editions added Hyper-Threading.
The Core 2 Duo processors have two Core 2 architecture CPU cores on a single die, often sharing a large 2nd level cache up to 12MB in size. They have a much broader range of speeds, mfg processes and applications than Pentium D and were commonly available in many FSB speeds from 533 to 1333.
All Core 2 Duo processors are 64 bit and none support Hyper-Threading.
When given the option, always choose a Core 2 processor, even if it has a lower Megahertz rating. Even the lower-branded Pentium Dual Core (essentially a Core 2 dual processor with a smaller, shared cache) above ~ 2.0GHz will best top end Pentium D's in processing and power efficiency.
Both are considered obsolete, as Intel is phasing out the Core 2 architecture in favor of Core i3/i5/i7.