Advice on memory for a Dell M4500

Hi all,

Currently have an M4500 with 4GB Nanya ram (1066 DDR3) and wanted to upgrade to 8GB in-time for an imminent reformat.

Bought a pair of 4GB Hynix 1066gb DDR3 after I read one person on the Dell 4500 thread had that from stock apparently and fitted it in today however...

First thing I noticed is on powering the machine on, there is a 5 second delay (each time) of just a black screen before the Dell logo appears.

I decided to boot into Windows initially to be greeted by a BSOD.
I then ran Dell diag and found a memory : Integrity test failed.

Having done some diagnostic work, it turns out this is only happening to one of the sticks so I currently have 1x4GB Hynix and 1x4GB Nanya.

So where do I go from here?

I will be potentially getting a full refund for both sticks rather than keep just the working one unless I get advice from here that I should just grab two new replacements?

I guess the questions are:

1. Do I get replacement pair and if diags say they are ok, they are ok
2. Do I go for another brand?

Bought them from a power seller from eBay and though the chips are indeed hynix, they do have a plain blue heatsink over them with no branding whatsoever on it.

Appreciate any help