Actually it will probably more than double. The Xeon 5160 can almost double two prestonias at 3.6 GHz and an E6600 is just a desktop version of the 5140 which shouldn't be too far off of dual 3.2 GHz
Actually E6600 is identically clocked to 5150 and will be faster than 5150 as chipsets to Core 2 are superior in performance to the Xeon chipset despite the fact that Xeon has 25% faster FSB.
Search out Tomshardware tests for Core based Xeon tests and with Quad-channel DDR2-533 memory, it achieves 5.8GB/sec and with Dual channel memory it gets 3.8GB/sec. Core 2 based platforms can achieve 5.4GB/sec on DDR2-667 CL5:
http://www.firingsquad.com/hardware/core_2_memory_tuning/page12.asp
Not to mention faster RAM for lower prices are available for Core 2 compared to Core-based Xeon.
Latencies are also significantly higher. I found searching over google that its around twice higher as the Core 2 systems.
SpecFP is 10% higher on the 2.93GHz Core 2 Extreme compared to 3GHz Xeon 5160.
It might be Core 2 Duo E6700 performs good as Xeon 5160 clocked at 3GHz.
In response to the first poster.
According to Toms(Link: http://www.tomshardware.com/2006/07/14/core2_duo_knocks_out_athlon_64/page13.html), E6700, the same CPU posted on the setup you want to buy compared to Pentium D 840, which is a dual core running at 3.2GHz and is roughly 2x Prestonia at 3.2GHz, performance advantage is summarized below:
3DS Max 8.0: 86% faster
Photoshop CS2(Rendering 5 pictures 66MB, 7 filters): 35%
Photoshop CS2(Converting 5 photos 9MP to 0.8 MP): 66%
Premiere Pro 2.0(Windows Media Encoder 9.1 AP, 24 sec HDTV 1920x1080 MPEG2 to WMV): 52% faster
You can think of Core 2 Duo performing twice as fast as equivalently clocked Pentium D's at same clock speed.
Since Prestonia Xeons use 533MHz FSB, you can expect the advantages to be much greater than above. You can derive whether its worth it or not. It does look quite a lot faster.