Thanks for the update Alex. Most new 'Nehalem'-based servers such as the model you're looking at use triple-channel RAM, notice how they have 'divisible-by-three' memory slots (3, 6, 9, 12, 18 etc) where previous models had 'devisible-by-two' versions (2, 4, 8, 16 etc). You can put less than three modules in at a time but you're likely to miss out on the full performance of the new 'QPI'-based memory architecture.
Also of great importance with these new chips is to balance your memory across processors - if you only have a single processor then you shouldn't put memory in the slots associated with the empty processor slot, only with the first processor - this effectively halves the memory capability of single-processor servers. If you have two processors you need to ENSURE that they both have the same amount AND type/size of memory as each other.
Now onto your actual question. With these new servers the more physical memory chips you use the slower they get - they're still faster than the previous generation of chips/memory even at their slowest but you should aim to use less of the most capacious modules that make sense to you financially.
Looking at the specific machine you want I would urge you to go with either 48GB Memory (12x4GB) or 24GB Memory (6x4GB) based on your requirements. Best of luck.