How important is using identical memory sticks on Intel server boards

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.

The impact is greater than if you were running an older CPU with no support for EPT, but realistically the only way to determine if it's going to affect your workload is to actually profile the workload. Don't take the hypervisor out of the equation, because the whole point is to test the setup that you're running, not some hypothetical benchmark figure. Ignore Memtest86+, ignore bare-metal OSes, just find a virtual machine that's representative of a memory-intensive workload in your environment and beat the crap out of it.

Guy is dead-on: most consolidated workloads are bound by the amount of memory in the system and not by any other resource. By decreasing memory contention, the extra memory will probably help you out by enabling you to use more of your memory-intensive VMs' RAM as cache instead of it being ballooned out under pressure.