hi Leonardo,
the problem with dual boards are:
- only for FB-DIMMs (more expensive and lower clockrates)
- not good for overclocking - only a few models have the options for clocking (i5000P chipset; socket 771)
Dual Quad xeon 2.0/2.66 GHz:
Intel Xeon DP E5335, 4x
2.00GHz, 333MHz FSB, 2x 4MB shared Cache, 771-pin boxed ~€650 x 2 = ~€1300
or
Intel Xeon DP X5355, 4x
2.66GHz, 333MHz FSB, 2x 4MB shared Cache, 771-pin boxed ~€1100 x 2 = ~€2200
ASUS DSBF-D, i5000P (dual Sockel-771, FB-DIMM PC2-5300F) ~€400
Kingston ValueRAM FB-DIMM Kit 4096MB (2x2048) PC2-5300F ECC CL5 (DDR2-667) ~€400
Good Powersupply - Seasonic M12-600 600W ATX 2.2 ~€130
A BIG quiet Case ~€150
videocard, harddisk(s), dvd-rw, winXP x64 ~€450
total:
E5335 2.0GHz, 4GB ~€2830 ~$3862
X5355 2.66GHz, 4GB ~€3730 ~$5090
i don't know what config you had chosen for $5600, but the apple price seems to be not so bad...
...or maybe a dual E5335 and clock it up to 3GHz
additional info:
http://forums.2cpu.com/