This is the information from the crucial website it seems, no?  I was confused by them in particular because they mention that the limit is 24gb but only offer a max of 12 gb....

The 12 GIG kit is 3 SLOTS.  12 + 12 = 24 GIG = 6 slots

Your Precision Workstation T3500 System Specs

System specs as shipped by the manufacturer.
Scan your system to view your specific system configuration.

Memory:

DDR3 PC3-10600, DDR3 PC3-8500, DDR3 PC3-12800 

Memory Type: DDR3 PC3-10600, DDR3 PC3-8500, DDR3 PC3-12800, DDR3 (ECC)
Maximum Memory: 24GB
Slots: 6
Each memory slot can hold DDR3 PC3-10600, DDR3 PC3-8500, DDR3 PC3-12800 with a maximum of 4GB per slot.*
*Not to exceed manufacturer supported memory.

Mixing Vendors, speed, and size is NOT RECOMMENDED.

Recommended Upgrade

Yes I get that, haha.  Just seemed kinda odd that there was no 6x4gb package.  But to the question at hand would such a configuration be compatible?

6 X 4 gig works fine.  You cannot however do 3 x 8 gig or even 1 x 8 gig.

There is a MAX SIZE per slot of 4 gig.

Thats why they say "with a maximum of 4GB per slot."

Thank you Osprey.  I just read a new article that stated this,

"I bought a Dell T3500. According to a post on the Dell forum, it's mobo doesn't support registered memory (it does support ECC memory). (No, they're not the same thing, so please don't post saying they are.)

But, according to a Word doc article done by NewEgg, if you're going past 4GB, you need registered memory. (You can find that doc by going to the Wikipedia article on registered memory and then clicking on the link. Then see their rationale.)"

I am very confused at this point, so which type of memory is needed when you go past 4GB ECC registered, ECC non registered, NECC?  I still cannot tell what one is needed.  Would unbuffered Non ECC 6x 4GB be compatible?

Yes, it matters.

Thank you for your replies.  Does it matter if it is ECC or NECC at the higher levels of ramm?