Trying to upgrade RAM 2008 XPS 420

Do we assume you're running a 64-bit version of Windows so you can use 8 GB?

Are you adding RAM to the existing or are you completely replacing the old RAM with 4 matching, new 2GB modules?

What color are the 4 diagnostic LEDs on top bezel when it won't boot? If any stay blue, look up error codes in the manual.

Unfortunately, some brands of RAM aren't fully compatible with Dell systems, even when the specs are correct. So it's possible what you got isn't compatible. Or one of them is defective, even if it works with less than 8 GB installed.

You can try this: After you install all 4 modules, remove motherboard battery and press/hold power button for ~30 sec. Then reinstall the battery (right-side-up!). That will clear BIOS and force it to see what hardware is installed. Note: always power off, unplug and press/hold power button for ~15 sec before working inside the case.  Then reboot.

And it's possible you have a bum slot on the motherboard...

EDIT: I fixed the link. :emotion-10:

I will try the BIOS reset and report back.

I am running 64 bit Windows 7 and all slots and all sticks work when only one is unplugged.

I'm getting 3 and 4 light up plus the Miniview display says memory is detected but a memory error has occurred. But again only when all 4 are in, regardless of their order. 

This is such a strange problem as I even did a crucial scan that said I could update with 8gb! Thanks for replies so far. :emotion-2:

The manual says you can use 4x2GB for total of 8GB, so I don't know what the other responder is trying to say. Can I assume you purchased the RAM from Crucial? If the BIOS reset doesn't solve the problem, then I suggest you contact Crucial to swap all 4 modules for you.