Is the CPU on the Power Mac G5 upgradable? How is it mounted?

Unfortunately, no. The processor, or processors, in the Power Macintosh G5 models cannot be upgraded.

The processors in all models are mounted on a custom daughtercard with a 300-pin connector. Although this daughtercard does not contain the ROM, it appears that the processor is controlled by Apple ASICs (Application-Specific Integrated Circuits), and consequently it is not feasible to upgrade the processor.

What is PCI-X? How is it different from PCI?

Apple explains that:

PCI and PCI-X cards enable the Power Mac G5 to perform specialized tasks, such as video acceleration and audio digital signal processing (DSP). PCI-X supports 3.3V signaling and Universal 33MHz and 66MHz PCI cards. . . Three 64-bit PCI-X slots let you add one card running at 133MHz and two cards running at 100MHz. Three 32-bit PCI slots allow you to add three 33MHz cards.

In simple terms, PCI-X is quite a bit faster than the earlier PCI standard.

Also see: Which Power Mac G5 models have PCI, PCI-X, and PCI Express slots? How many PCI slots of what speed does each Power Mac G5 have?

What is PCI-Express (PCIe)? How is it different from PCI and PCI-X?

PCI-Express (PCIe) is provided by the Power Macintosh G5 Dual Core (2.0), Power Macintosh G5 Dual Core (2.3), and Power Macintosh G5 "Quad Core" (2.5).

Apple explains that PCI-Express is:

A modern industry standard sponsored by the Peripheral Component Interconnect Special Interest Group (PCI SIG). Because older parallel technologies placed multiple devices on a single bus, the slowest device determined the speed of the entire bus. A serial technology, PCI-Express guarantees each device dedicated bandwidth to and from the system controller.

Apple also notes that PCI-Express:

Communicates in 250-MBps 'data lanes.' PCI-Express cards and slots are defined by their bandwidth, or number of data lanes--typically one lane, four lanes, eight lanes, or 16 lanes. At 250 MBps per lane, a four-lane slot can transfer data at up to 1 GBps and an eight-lane slot, up to 2 GBps--approximately twice as fast as a 133 MHz PCI-X slot.