[CentOS] Memory vs. Display Card

Since memory has become quite cheap lately I decided to move from 2 GB
to 6. When I installed the memory every thing was fine until I went to
run level 5. At that point the screen turned to garbage and the system
froze. Is there a way to fix this so I can use the memory I bought? Do
I need a new display card?

Current hardware:

  Intel D975XBX2 Motherboard
  VGA compatible controller: ATI Technologies Inc RV505 [Radeon X1550 64-bit]

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