Usually a monitor and keyboard and mouse would be kept. Your HDD could be kept (unless u think it's to small for u).
What u have to change, I think, is the CPU, mobo, RAM(DDR 2 is out of date now.), Graphics and PSU (if u play large games), HDD (if u required). OS (unless u have a retail version of yourself)
Could u tell us what parts u would like to keep. Your case only, or there are other parts u would keep? (I am 90% sure that u keep the monitor, HDD, keyboard and mouse, just to make clear to us.)
Usually a monitor and keyboard and mouse would be kept. Your HDD could be kept (unless u think it's to small for u).
What u have to change, I think, is the CPU, mobo, RAM(DDR 2 is out of date now.), Graphics and PSU (if u play large games), HDD (if u required). OS (unless u have a retail version of yourself)