2 million for just the building the tower was an additional cost
No. A Pentium Dual Core is a cost-reduced version of a Core 2 Duo. Think of it as the new equivalent of a Celeron. The Pentium D is basically a dual-cored version of the Pentium 4, but is far less efficient than a Core 2 Duo (or a Pentium Dual Core).
Celeron doesn't refer to any particular processor series. Celerons were cost-reduced versions of their Pentium equivalents. The Celeron in question could be based on a Pentium 2, Pentium 3, Pentium 4, Pentium D, or Core 2 Duo. In which case the answers would be "Pentium 3, Usually Pentium 3, Celeron, Celeron, and Celeron", respectively.
No. It is far more efficient. Essentially, it is a cost-reduced version of the Core 2 Duo, but it is still more powerful than any Pentium 4.
The Pentium D is essentially a rebranded and double-cored version of the Pentium 4. The Core 2 Duo is a much more efficient and powerful processor.
The Pentium is not a risk architecture.
The cost would depend on the processor you wanted to purchase. An upgrade is probably possible (the Pentium Dual-Core is based on the Core Duo and Core 2 Duo processors) but you would need to make sure your motherboard supports the proper FSB speeds for your processor of choice.
The Intel Pentium line encompasses many processors of many different architectures ranging from the original Pentium Processor of 1994 up to the Core 2 Duo based Pentium simply called the "Pentium". The Intel Pentium used to be Intel's flagship processor. Recently this position has been taken by the Core 2 Duo and Core i7. The Pentium now is reduced to a budget processor that offers lower performance but also lower cost.
Pentium Duo Quad, Pentium core duo, Pentium D, Xeon, Itanium, Pentium M, Pentium 4, Celeron, Pentium 3, Pentium 2, Pentium Pro, 486, 386, 286.
No. The Intel Core 2 Duo is much faster than the older Pentium 4.
No, it will not. Pentium 2 fits two sockets (with adapter for one of them) 350 and 370. When Pentium 4 fits three different sockets 423, 478, and 775.
There is no Pentium 5 processor. The mainstream (non-budget) Pentium line ends with the Pentium D, which is essentially a dual-core Pentium 4. The Core Solo, Core Duo, Core 2 Duo, and Core 2 Quad all have a very different architecture from the Pentium 4.