Optiplex GX520. Refurbed, Win 7.
2- 2GB sticks memory
With both sticks in, reboot takes about 15 minutes. It sits on the black Windows splash screen, just before the mouse pointer and CTRL ALT DEL shows up.
With either one stick in, IN EITHER SLOT, booting takes a normal 1-2 min.
Sticks can be reversed in slots, singly or in pairs and there is no difference to the problem.