RAM Upgrade = Vista freezes

[gtharea]

I have a Dell Dimension 8300 with 1.5gb RAM. I have 3 DIMMs, two are 256mb and one is 1gb. They are all DDR 400mhz. Now, I have replaced the 256mb DIMMs with 1gb DIMMs(DDR 400), so i now have a total system memory of 3gb. I start my computer up, it loads the Vista welcome screen, i click my user icon, it says 'Welcome', and then my computer freezes. Sometimes it stays frozen, other times it automatically restarts. It is impossible to log on with this memory upgrade. If i switch back to my 1.5gb configuration, everything works just fine.

What is wrong? Is the memory bad? Any help would be appreciated

Oh, and i have Vista Ulitmate 32bit

 

[Tmagic650]

Make sure that the memory is the same make and speed. Even different chip configurations on the memory modules can cause trouble. Do you get errors when only using 2 memory slots? Have you checked with Dell to make sure that you have the proper memory for your computer?