sorry for the tough questions but this is how we techies troubleshoot. at least i do. find out exactly what each components specs are, determine compatibility, test for stability, and so forth then go from (components have to play nice together) there.
by the way, dell will only tell you what their recommendations are based on their supplies at hand. they only want to sell you their stuff. but that's a whole different thread.
i currently use this
program to identify my existing modules. however, it will only pick up your installed stix and not tell you what your mobo supports. also, forget about activeX controls via websites like crucial.com. 9 out of 10 the activeX control is wrong and mislead consumers. you are 100% accurate if you either look at your mobo manual or call the mobo manufacturer directly.
finally, there are many variables when it comes to selecting memory. from size to speed to voltage to type and so on.
MEMTEST86 is a great tool to use. the software is free and it will tell you what stix is bad and so forth.
well good luck. i hope my wealth of info is valuable to you.