xps 7100 memory upgrade problem

I have a Dell xps 7100 with 4 expansion slots.  I had 2 X 4mb in slots 1 and 2, with 3 and 4 empty.  I have had no problems. using PNY 4GB,DD3 DIMM,H PC3-10666.  I bought 2 more identical memory sticks and installed in slots 3 and 4.When i boot up the computer, I just get a blank screen--The hard drive gets some hits, then stalls out--no error message--no beeps--nothing.  When I pull out the sticks in 3 and 4 everything boots up and runs fine----Any Ideas as to what could be wrong??