Nehalem has 3 channels and I'm sure by now, most people would know the following facts about QPI performance:
a.6.4GT/s (DDR3-1333Mhz) for 1 DIMM per channel
b.5.86GT/s (DDR3-1066Mhz) for 2 DIMMs per channel
c.4.8GT/s (DDR3-800Mhz) for 3 DIMMs per channel
8GB DDR3-1333Mhz DIMM is extremely rare and so much more expensive than 4GB DDR3 at the moment.
So which one would give better performance? Let's cap the capacity at 24GB:
1x W3570 3.2Ghz 6.4GT/s + 6x4GB DDR3-1066Mhz (2 DIMMs per channel)
2x X5550 2.66Ghz 6.4GT/s + 6x4GB DDR3-1333Mhz (1 DIMM per channel x 2 processors)
2x X5540 2.53Ghz 5.86GT/s + 12x2GB DDR-1066Mhz (2 DIMMs per channel x 2 processors)
or some other 3 DIMMs per channel with slower processors combination.
Among processor speed, QPI speed & memory capacity, which is the most important factor? Under similar cost comparison?
Would getting bigger capacity DIMM, adding more DIMM modules or upgrading to faster CPU/RAM more cost effective to achieve better performance in the future upgrade within a server?