Inspiron 620 MT - Single or dual rank memory

Single rank or bank modules are usually faster. I don't know if it's still true, but there used to be a limit in the number of banks a computer could access. The dual rank modules use smaller (read older, more power hungry) chips. DO NOT mix modules with different ranks.

Older systems cannot use Dual or Quad Rank ram.

This is a Chipset and bios limitation.

http://www.crucial.com/usa/en/compatible-upgrade-for/Dell/inspiron-620

This vendor has guaranteed compatible ram

 http://www.memorystock.com/memory/DellInspiron620620s.html

Thats why LOW Density Ram is required for MANY Intel Chipsets.

Any ole joes crab shack ram isnt compatible.  If you get it from cruical or from dell its guaranteed to be compatible.  The speed you need depends on which cpu you have and its FSB.  Ranks count also you cannot mix single rank and 2 rank and 4 rank ram.  Its not required but being all from the same vendor for the sets of 3 helps. There are also processors that the motherboard supports that do not support ECC ram.

 YOU CANNOT USE 8 GIG Dimms ever. Nor can you use Registered and or Buffered Ram. Crucial doesn't seem to show 4 gig dimms working either.

I am upgrading to 8GB RAM. This will be 2x4GB of DDR3 PC3-10600 memory. For performance, which is better, single rank or dual rank memory. Just how much difference is it likely to make?

Thanks for the reply. I don't think this machine fits the 'older' profile. I believe it supports Dual Rank Ram. The setup guide (page 60/61) doesn't say so explicitly, but the service manual (page 38) shows that the system settings can be "Single" or "Dual". I checked my settings and it is reporting Dual - it's not an editable field. Those manuals also confirm the chipset as H61.

I followed the link for the crucial website, and when clicking 'View more compatible memory upgrades' it does in fact offer 4GB Dimm support. Following the 2x4GB link, irritatingly the specification does not specify the rank for this memory. However, googling the product key takes me to here, which does specify rank as 2.

Now, as far as I can see, neither Crucial or memorystock offered 1R memory as an option for upgrade. However, following the memory selector on Kingston lead to this. This is documented as single rank memory.

So, I'm still left with the original question, what are the performance characteristics of using single vs dual ranked memory for this machine?

Single rank is faster than two or four.

What you are referring to in the Service Manual is NOT rank but Channel. Dual channel is automatic and operational if there are matched pairs of modules in paired sockets. Single channel is a single module or unmatched modules. Dual Channel is faster than singe channel. Rank is the number of groups (for a better word) on a module. Dual channel is often dual sided modules using low density (cheaper) RAM chips that require separate addressing by the processor. You can not mix single and dual modules, and using dual rank nodules often limits the amount of RAM possible. Dual rank modules are usually used in servers that have huge amounts of RAM, but can also address lots of RAM. 

LOW DENSITY and Chipset north bridge limitations are the issue.  This is why 16 or 32 gigs is not possible. You also cant mix ranks.

In later cpus like the I7 the north bridge and video are integrated into the processor.

Fair point on channels, my error. On the topic of density, I note this is not quoted on so many occasions, both on the big vendor sites and elsewhere. Also, I note some are of the opinion that the definition of density is imprecise, with what is now regarded as low density, may have been regarded as high in the past.

What is the clear specification for this PC, for what constitutes compatible LOW density RAM and likewise the specification for incompatible HIGH density RAM?

This is not as complicated as being presented. Most computers are set up to support a certain number of Ranks.. Four slots of RAM means four ranks are supported, and the modules are built for this  So if you use dual rank chips, you  reach the limit with half the slots filled.

In addition, dual rank chips were designed for servers, where scads of RAM are necessary, and dual rank chips allowed double amounts of memory on each board. I have never seen a spec for a non-server computer to use dual rank chips. There is no reason that system boards couldn't be built to accommodate dual rank, but I think being able to use either type would complicate the board.  In addition, at least in the past, most dual rank modules offered for sale at low prices were USED, having been pulled from old servers. High density single rank modules are not that expensive

4 slots does not mean 4 ranks are supported.  If they were 32gig would be possible in a GX620 mini tower.  This is why AMD works with High density and INTEL doesn't. Ranks are the number of chip selects per channel.  The GX620 DOES NOT AND WILL NOT Support 4 rank modules EVER. The Fully Buffered DIMM (FBDIMM) is an alternative to using 4-rank DIMMs to support high density memory capacity. FBDIMMs solve the stub bus timing challenges and can increase system memory capacity by allowing more DIMMs sockets in the system when physical space is not an issue (ie: high-end servers use 8 to 16 DIMM slots). Apple does this on the mac pro with Riser cards.Dell does that too for more than 8 slots. 4 rank modules,  approved by JEDEC, address this gap by allowing up to 72 DRAMs per memory slot, enabling the 32GB per CPU capacity goal using commodity 512Mb DRAMs. When the 1Gb DRAMs are finally in mass production, 4 rank modules double the reach again to 64GB per CPU.