In a fair test, it's unlikely that the even the 3.0Ghz Quad Xeon is near 2.1 times as fast as the G5 Quad.
Apple's actually in a tough spot. Throughout the rest of the line, the switch to Intel dual-core processors brought about an undeniable boost in CPU performance (except the single-core mini).
I'm convinced the Mac Pro (2.66 and up) is faster than any previous Mac, just not by the kind of factors they were able to claim with the switch from single to dual on all other models except the cheapest mini. The lack of standard Airport and Bluetooth is the same as on the dual-core G5, except the price of the AP/BT option has gone down to $79.
Going to full 64-bit in Leopard will not result in any performance boost. That's more marketing BS. Most applications don't need, can't use, and won't benefit from, full 64-bit.
Quad G5 2.5Ghz 4.5GB 2x250G, PB 15" 1.5Ghz,80G,1.5G Mac OS X (10.4.7) Logic Pro 7.2.1, Focusrite Saffire...