As a rule, you may not use ECC and non-ECC memory together as well as registered and un-registered RAM.
Also, you may not mix RAM voltages.
They must be uniform in those specs, as the memory controller will be confused on how to treat the ram. There can be minor variances in the timings, but usually the system will select the slowest of the ram sticks' reported speeds to run them all at.
This applies across most computer systems and is not limited to the Dell T1700.
Proverbs 4:7 - Wisdom is the principal thing; therefore get wisdom; Yea, with all thy getting get understanding.