While I wouldn't recommend eating one, Intel processors are manufactured to be RoHS-compliant, meaning they contain virtually no lead or mercury. They won't poison you, but they'd definitely cause some intestinal obstruction.
It could be changed. P4 and Celeron D are using the same socket - 478 or LGA 775. You should check your motherboard's chipset compatibility for further info.
The Intel Pentium D uses the older, hotter, and slower NetBurst architecture of the Pentium 4. It would not be a good idea to invest in one for a new build. A Pentium Dual-Core would be faster, run...
It's often difficult to compare the performance of two processors, especially when one is much older. Depending on the type of computations you are doing, you may see anything from no change to a...