gistfile1.txt

MAC PRO
MB:    dual socket nehalem xeon
CPU:   2.66 Ghz Nehalem Xeon
MEM:   3GB 1066MHZ DDR3 ECC
HDD:   640MB 7200RPM SATA 3G/s
VID:   Geforce GT 120
CDR:   18x SuperDrive
Input: Apple Mouse + Keyboard
OS:    Mac OSX 10.5
CASE:  Mac Pro Aluminum Case

Price: $2,499.00

Newegg

MB: $479.99
ASUS Z8PE-D12(ASMB4-IKVM) Dual LGA 1366 Intel 5520 Tylersburg SSI EEB 3.61 Dual Intel Xeon 5500 Series w/ Remote Management Server Motherboard - Retail
CPU: $979.99 
Intel Xeon X5550 Nehalem 2.66GHz LGA 1366 Quad-Core Server Processor Model BX80602X5550 - Retail
MEM: $98.97
Crucial 1GB 240-Pin DDR3 SDRAM DDR3 1066 (PC3 8500) ECC Unbuffered Server Memory Model CT12872BA1067 - Retail (x3)
HDD: $79.99
Western Digital Caviar Black WD6401AALS 640GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
VID: $83.99
ASUS EN9500GT TOP/DI/512M GeForce 9500 GT 512MB 128-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Retail (Equivalent to the nVidia GT120)
CDR: $21.99
LG Black 20X DVD+R 8X DVD+RW 16X DVD+R DL 20X DVD-R 6X DVD-RW 12X DVD-RAM 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM 2MB Cache SATA 20X DVD±R DVD Burner w/ SecurDisc Tech - OEM
Input: $14.99 + $9.99
OCZ OALKBELX2US USB Wired Slim Alchemy Series Elixir II Keyboard - Retail
Microsoft Comfort Optical Mouse 1000 - OEM
OS: $0
Ubuntu
Case: $249.99
LIAN LI PC-V1200Aplus II Silver Aluminum ATX Mid Tower Computer Case - Retail
PSU: $325.99
ENERMAX REVOLUTION85+ ERV1050EWT 1050W ATX12V / EPS12V SLI Ready CrossFire Certified 80 PLUS SILVER Certified Modular Active PFC Power Supply - Retail (The estimated power of a mac pro PSU is 1010 Watts)

Price: $2345.88 (+ shipping) 

Difference between the two: $153.12