Its not about the CPU speed. Its about the motherboard bottlenecking the entire computer for gaming.
For starters, the motherboards that support Duel Xeon 771 Do not support SLI (link below of Duel Socket 771 Motherboards). So now, our duel CPU computer is already limited to 1 graphics card. The advantages of the server style motherboard are more ram, and two CPUs. Both of which will never make up for the fact that your running 1 graphics card. This has be proven when a single 8800gtx was put with a kentsfield (more or less the same as clovertown) and the CPU bottleneck was not hit (even with dry ice cooling and overclocking) until the second graphics card was added. The Builder then stated "it's all CPU bottlenecked now" (link below). It addition to the graphics card issue, the server motherboards that handle the Xeon CPU's are not the greatest at overclocking, usually reaching slower speeds then the LGA775 boards. Server boards have more ram, but in gaming, it not important because they are running at slower speeds, typically 667. A game is bottlenecked by speed of the ram, not the size. So for the typical gamer with tons of money, a single CPU solution would be much more desirable then a Duel CPU. I can think of many cases that I'd rather have a duel server style computer, but gaming is not one of those cases.
VR-Zone : Technology Beats - Overclocking the EVGA 8800GTX ACS3
Newegg.com - Buy Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!
Most xeons are LGA775 now, and run a 1066 bus, the same as most core 2 duos, eg:
Intel Xeon 3200 Series X3220 "LGA775 Kentsfield" 2.40GHz (1066FSB) - Retail
Also, these days, Xeons are usually the SAME CPU anyway, just sold as "Xeon" with a higher price tag. What actually is the difference between that xeon ^^^ and this:
Intel Core 2 Quad Pro Q6600 "LGA775 Kentsfield" 2.40GHz (1066FSB) - Retail
Only the fact that the Xeon is rated for lower volts.
While it is true a Dual CPU board may not be quite as good, you don't have to run one. Just stick it in a supporting LGA775 board.