I have looked at all the similar postings and nothing quite explains my problem.
I have a dell OptiPlex GX520. I checked the specs on the dell website and also ran your scan programme and both indicated 4gb would work. Unfortunately it won't work properly. I am rebuilding the machine with a new hard drive. When I tried to install windows it froze at the same point twice. I put the old memory back in and it loaded up OK. I then installed the memory and every time I boot up it freezes just after the logo comes up (Windows7). When I put back the old memory it works OK. I have looked at the Bios and the memory specs are the same and it recognises there is 4gb. I have tried each of the memory cards on their own and both work OK but they will not work together.
One of the other tyhreads talked about low density memory. I have looked at the BIOS and the old and new are reporting 8x
I am sorry for the issues that you have been experiencing with your upgrade. The modules have part numbers on the front label that will say something like CT25664AA667.M16FH.. What are the part numbers on the modules that you have ordered? Thank you in advance for this information.