1) That's an overclocked P4
2) While it can technically hold that much ram it can only use it through a funky slow thing called PAE, 'cause it's not 64 bit
3) Yes that machine would be faster than a G5 if it existed (by quite a bit, on most things, the P4EE is pretty quick), but it'll cost you $800-1000 per chip (hence the $7500+ price tag), and probably be less reliable 'cause of the overclocking
4) If you really want to see how much BS this thing is, check out the number of "optional" PCI slots they claim you can fit in a 70 pound machine...