The following forum post might help.

https://www.dell.com/community/Desktops-General-Read-Only/Optiplex-380-RAM-Upgrade/td-p/3609245

https://www.dell.com/community/Desktops-General-Read-Only/Dell-Optiplex-380-max-RAM/td-p/4720466

According to pg. 11 of the tech guide, the max total RAM for the 380 is 4GB @ 1066MHz.

Optiplex 380 Tech Guide

Actually, the max is 8gb RAM provided a 64 bit OS is installed, but it seems from other posts that people using only a 32 bit OS were able to make 8gb RAM work after updating the BIOS. It seems my problem is that the sticks I bought are 1.5v, and 1.35v is the max. Thanks for the help everyone.

I have 380 MT,

i tested a whole box of DDR3 just this last week, to end this myths told everywhere. it seems

first off , got newest BIOS>? loaded< so it can understand more DDR types?  (make it smarter first)

rule #1 it is G41 intel mmc,  it supports both DDR2 and DDR3, (not dell) but does, and some mobo have 4 slots, 2 are DDDR 2 and 2 are DDR3, making others reading other forums read about those other PCs and get CORN-fuzed.

THE 380 is 100% DDR3 , and for that 1 rule is 1/2 max memory, clearly stated in the G41 chip set data sheet,

now the bad news, new memories do not work in this PC< 4g, each up. size , as seen at BESTBUY, sorry.

it only runs OLD RAM>  DDR3. that is limited by this page in the G41 spec. lets look at that first. and learn.

the best RAM sticks are 2G, and with 16 chips NO LESS chips  and 2 ranks. this is limit of the G41 ! not DELL.

sure dell may have other BIOS  limits IDK, they dont spec it out. (G45 PC far better)

all my 2GB sticks failed, vast,

only my oldest 16 chip sticks works

and 1GB also failed in my box of stick until I go it right in the above list. the big start read.  128x8 chips

there is no voltage problem , for example Samsung still makes the 128mx8 chip and the data sheet clear as day states the 1.35v chip is backwards compatible to the (380) 1.5vdc motherboard state 10 times in the datasheet.

and the other top three? same words.

this 380 is a 1.5v DDDR3 PC,

yes this G41 is  a pain, so upgrade to PC with G45 and get 4 real slots? of DDR. heck go Z170 !

long low beeps indicates your ram is not and never will be compatible.

You cannot use 1.5v high density dimms.

8 gigs total is max using 1.35v dimms

DDR3 PC3-12800 • CL=11 • UNBUFFERED • NON-ECC • DDR3-1600 • 1.35V • 512MEG X 64 •

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-380-desktop

When I went to Crucial website and search for memory for Optiplex 380 , all the memory it showed me was 1.5v , only after I downloaded , and installed Crucial system info program did it show me 1.35v memory , that is because I already had 1 dimm of 1.35v memory installed. It was trying to match my pre-existing memory as closely as possible . I also did the same search on 2 other memory sites , and they both showed me all 1.5v memory. It is clearly backwards compatible. 

It's been almost a year since I made my post above.  It simply states what the ol' Tech Guide reports - 4GB RAM is the max.  However, knowledge has changed since then.  Speedstep also reports 8GB RAM is the max.

Here's a link to another thread in which the OP wanted to upgrade RAM on a 380 SFF.  https://www.dell.com/community/Optiplex-Desktops/Optiplex-380-sff-max-ram-4gb-or-8gb/m-p/7513690#M40051 

Also, according Dell's own Optiplex 380 Spec Sheet , 8GB is the max RAM for the 380.