Intel Xeon 6134 + One DIMM per channel or two DIMMs per channel for maximum memory bandwidth

Wikipedia has a fairly good explanation of rank (link). I'd say RamCity (a vendor for Kingston memory) has a more succint explanation on ranks (link):

A memory rank is, simply put, a block or area of data that is created using some or all the memory chips on a memory module.

A rank must be 64 bits of data wide; on memory modules which support Error Correction Code (ECC), the 64-bit wide data area requires an 8-bit wide ECC area for a total width of 72 bits. Depending on how memory modules are engineered, they can contain one, two, or four areas of 64-bit wide data areas (or 72-bit wide areas, where 72 bits = 64 data bits and 8 ECC bits).

The article goes on mentioning price variation:

Why do the single- and dual-rank memory modules vary in price?

In general, single-rank memory modules are built using x4 (“By 4”) DRAM chips and are more expensive than dual-rank memory modules (which are built using x8 DRAM chips); both module types have the same number of chips but the x4 DRAMs are more expensive than x8 DRAMs. Dual-rank memory modules may limit future upgradeability and capacity of servers when using PC2700 or PC2-3200 memory. This tradeoff between memory cost and capacity is important to consider when purchasing memory modules for Intel Lindenhurst-based servers.

In terms of performance, I'd refer to wikipedia:

The ranks cannot be accessed simultaneously as they share the same data path.

So to sum up everything, it appears that ranks have more to do with density and pricing than actual performance. Granted, I'm working off of generalized statements from a vendor and wikipedia, I don't think most people put much effort into researching ranks. All that matters (for most server admins) is that RAM have matching ranks. I don't think it's an actual specification or requirement but it helps keep some consistency and keeps memory interchangeable within a number of similar servers.

Keep in mind that most servers are upgradeable and RAM density has a large part in factor. It's best (albeit more expensive) to get the more dense RAM for servers to make room for future upgrades.

So you're talking DL385 G7 vs. DL380 G7 right?

The most important question you need to ask is if this is to be part of an existing or future DRS cluster - if so then the manufacturer needs to match across the cluster so that should override any speed gain or loss.

If you could ONLY buy one or the other of these two chips I'd go for the AMD, simply because 16 full cores (i.e. 2 x 8 in the 6134) will give you overall better performance than the 5620's 8 full cores plus hyperthreading - simply because not every task will make best use of threading. Plus the 385 has 24 DIMM sockets over the 380's 18. That said if I had a choice I'd go for the 12-core 6172 chips to be honest, their power-per-watt and price is pretty compelling.

Oh and the 6134 is 2.3Ghz by the way.

NB: - It's worth you knowing that I'm a known Intel advocate but I'm more than happy to recommend AMD here.