You cannot use BUFFERED/REGISTERED Ram EVER.  You must use DDR2 ECC Unbuffered RAM

This type of ram is VERY specific and its LOW Density INTEL compatible ram.

Only Ancient Vacuum Tube ECC models work however.  Its ECC and its 4 gig and its NOT Buffered/REGISTERD  You cannot mix them.

Don't make the mistake of buying High Density or FULLY BUFFERED ram and expecting it to work.

Memory Type:DDR2 PC2-6400E (ECC)

Maximum Memory: 16GB Slots: 4

Each memory slot can hold DDR2 PC2-5300, DDR2 PC2-6400, DDR2 PC2-8500 with a maximum of 4GB per slot.

You cannot Mix ECC with Non ECC and the ram should all be matching from the same vendor unless you are looking for trouble.

You cannot take advantage of  1066 Mhz.

It will work with non ecc.  Here is a vendor that guarantees it to work.

http://www.memorystock.com/memory/DellPrecisionWorkstationT3400.html

The issue I would have is that its soo expensive its easier to buy a used T3500 which uses DDR3 and the Ram is Cheaper as well.