Hi GaryRr, Check and make sure the 512 sticks are properly seated in the computer. If it still dosen`t work, it may be that they are not compatable with the new sticks of ram.
I have a “hand-me-down” Dell PowerEdge 400SC, which had two 512MB sticks of Patriot memory (p/n PDC1G3200LLK), which Patriot euphemistically calls a “1GB kit.”   I decided to upgrade the memory, and bought two CORSAIR 1GB 184-Pin DDR SDRAM DDR 400 (PC 3200) Desktop Memory Model VS1GB400C3  memory sticks. When they arrived I removed the two Patriot sticks, and inserted the Corsair ones into the same slots I had removed the Patriot memory from (Channels A&B DIMM1). Turned on the computer; it registered the new memory fine.   Then I figured I could insert the two 512MB Patriot sticks in Channels A&B DIMM0, and end up with 3 GB. No such luck. As soon as I turned the computer on, I got a long beep, and just the black screen.   What am I missing?  GaryRR
Memory upgrade problem