I'm kind of ashamed to admit that I posted about this a year ago and never acted on it for lots of reasons...but now I'm installing an SSD and decided to upgrade the RAM as well. I definitely have 9 GB (2 + 1 X 3). Knowing how things change I thought I'd best get some new guidance because RAM is not one of my areas of expertise at all. I want to max out (24 GB) unless anyone thinks that's a bad idea. I do no gaming but a lot of photo/video editing and ProTools work. More than anything I want the system to be stable (no OCing).
I used the Crucial scanner (which I used to believe one could trust) and here's what it reported:
Memory Type: DDR3 PC3-12800, DDR3 PC3-10600, DDR3 (non-ECC)
Maximum Memory: 24GB
Currently Installed Memory: 9GB
Total Memory Slots: 6
Available Memory Slots: 0
But I checked speccy and here's what that said:
Slots #1, 3, 5
Type DDR3
Size 2048 MBytes
Manufacturer Hyundai Electronics
Max Bandwidth PC3-10700 (667 MHz)
Part Number HMT125U6BFR8C-H9
Serial Number 1946B7A2
Week/year 37 / 10
SPD Ext. EPP
Slots #2, 4, 6
Type DDR3
Size 1024 MBytes
Manufacturer Hyundai Electronics
Max Bandwidth PC3-10700 (667 MHz)
Part Number HMT112U6BFR8C-H9
Serial Number 04C3E581
Week/year 48 / 10
SPD Ext. EPP
So I don't know whether I have PC3-10700 (667 MHz) or a combination of DDR3 PC3-12800, DDR3 PC3-10600. 
The Crucial scanner recommended this:
16GB kit (4GBx4), Ballistix 240-pin DIMM, DDR3 PC3-12800 memory module
Part Number: CT3587496
Module Size: 16GB kit (4GBx4)
Package: Ballistix 240-pin DIMM
Feature: DDR3 PC3-12800
Specs: DDR3 PC3-12800 • 8-8-8-24 • Unbuffered • NON-ECC • DDR3-1600 • 1.35V • 512Meg x 64 • Low Profile •
I find this really weird because if I have 9 GB (2 + 1 X 3) that means it's tri-channel (why would they recommend only 4 sticks?).
Based on the system make/model, NewEgg's memory finder recommended this:
Dell 4GB 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Desktop Memory $32
Dell 2GB 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Desktop Memory $25
I assume they mean X3, for 16 GB total = $171
I'd really rather go with a name: Corsair, Patriot, Kingston, than a Dell-branded stick.
I found this on TigerDirect:
Patriot G2 Series PGD38G1600ELK Division 2 Edition Desktop Memory Kit -
8GB (2 x 4GB) PC3-12800, DDR3 1600MHz, 9-9-9-24 CAS Latency, XMP Ready
$45 X 3 = $135
If someone could guide me on the spec I actually need I'd greatly appreciate it.
I'm not gun shy about most specs, but RAM shopping always makes my head hurt. 
dg