Build a comparable Dell Precision (2 cpus) The dells are actually more expensive than the equivalent Mac Pro.
from the Dell site:
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My System Details
BASE Dell Precision T3500, CMT, 85 Percent Efficient Power Supply
OPERATING SYSTEM Genuine Windows® 7 Professional Bonus- Windows XP Professional downgrade
ENERGY STAR No Energy Star
PROCESSORS Quad Core Intel® Xeon® W3520 2.66GHz, 8M L3, 4.8GT/s, Turbo
CHASSIS CONFIGURATION Mini-Tower Chassis Configuration
MEMORY 3GB, 1066MHz,DDR3 SDRAM, ECC (3 DIMMS)
WARRANTY & SERVICE 3 Year Basic Limited Warranty and 3 Year NBD On-Site Service
VIDEO CARD 256MB ATI FirePro V3750, DUAL MON, 2 DP & 1 DVI
HARD DRIVE/RAID CONFIGURATION C1 All SATA drives, No RAID for 1 Hard Drive
HARD DRIVE INTERNAL CONTROLLER Integrated Intel chipset SATA 3.0Gb/s controller
750GB SATA 3Gb/s with NCQ and 16MB DataBurst Cache
OPTICAL DRIVE 16X DVD-ROM with Cyberlink Power DVD
MONITOR No Monitor
FLOPPY & MEDIA READER No Floppy Drive and No Media Card Reader
RESOURCE DVD Resource DVD - contains Diagnostics and Drivers
My Accessories
SPEAKERS No Speaker option
KEYBOARD Dell QuietKey Keyboard
MOUSE Dell USB 2 Button Optical Mouse
SYSTEM DOCUMENTATION Documentation, English, with 125V Power Cord
My Services & Warranties
ALSO INCLUDED WITH YOUR SYSTEM
Quick Reference Guide Quick Reference Guide, English
Shipping Packaging Options Shipping Material for System
Windows 7 Upgrade Program Info Windows 7 Upgrade Web Site
Starting Price $1,864
Instant Savings $241
Subtotal $1,623