Re: as long as I go for all 4 of the same type (e.g. DDR3 PC3-12800), then they would be compatible, possibly just running at a slower speed than they might be capable of? go for the least expensive major name brand pc3-10600 aka ddr3-1333 on eBay such as Samsung Elpida Crucial Micron Hynix.  These older Dell models are not picky of exact name brand, as when you bought pc from Dell originally they may ship with any of these major name brands. Some users say avoid Kingston which is a cheaper brand. 12800 may or may not work. There are a lot of used but still perfectly working pc3-10600 on eBay so no need to take your chance on 12800.
I have recently started using Photoshop Elements - which can use a lot of memory. Out of interest, I looked to see how much a memory upgrade would cost.  This machine no longer appears on the list for upgrades provided by Dell, so I looked at a couple of alternative suppliers.  On one site, it suggests "DDR3 PC3-10600 1333MHz 240-pin DIMM".  Looking at the Crucial site, it suggests "DDR3 PC3-12800". On the Crucial site, there appears to be no mention of the frequency.  However, when I did a search on eBay for the same product number, I found some modules that were described as 1600MHz.I am not technical, so don't know what it all means.  However, I'd like to know whether the 10600 and 12800 designations would have any impact on compatibility  (both with the computer and with existing memory modules if I only replace two out of the four).  Similarly for the 1333MHz vs the 1600MHz.Given the age of the machine, I was hoping I might be able to pick up some replacements from eBay without spending a fortune, but am confused with the numbering and concerned about compatibility.  Can anybody shed some light on the matter?Thanks.
Studio XPS 7100, memory (RAM) upgrade