Problem mixing two similar memory modules

I am new to this forum. By the way, sorry for my horrible English.
I have a Dell 530s with 2 GB of ram (2 x 1 GB) and I bought two additional modules (up to the maximum: 4 slots x 1GB). Apparently new and old of different brands but all the same technical characteristics: 1 GB each, DDR2-666 MHz PC2-5300U CL5-5-5-15.

Initially Vista-32 bit said that the 4 GB were correctly installed, but some days later I began to suffer unexpected system restarts. Then I searched the web looking for the details of the message I received, and most of the similar cases pointed to ram among other possible causes. Memtest gave immediately some errors so, I removed the new ram and the problem disappeared.

Now, I still have the opportunity of changing the modules at the shop, but I should do it tomorrow or the day after tomorrow. So, if someone could help me so quickly, I would appreciate any advice.

Details of the old modules: Hinyx(Hyundai) HYMP512U64CP8-Y5 DDR2 1GB/64 @ 667 MHz 16x(64Mx8) CL5-5-5-15
New modules: Corsair VS1GB667D2 DDR2 1GB/64 @ 667 MHz 8x(128Mx8) CL5-5-5-15

I guess all of them should be 64M and not 128M. That's the only difference I can observe. If that was the case, alternatives available at the same shop are:
1) Buffalo DIMM 1 GB DDR2-667 Valueline (64Mx8) CL5-5-5-15
2) Dolphin DIMM 1 GB DDR2-667 (64Mx8) CL5
3) takeMS DIMM 1 GB DDR2-667 (64Mx8) CL5-5-5-15

Price makes no difference. I would appreciate a quick answer

As long as the pairs are appropriately socketed in the correct banks, then there shouldn't be a problem with the mixed brands.

I would suggest you reinsert the 'bad' modules and make very sure they are installed correctly and the locking tabs click into place-- it may be that one went in a little cockeyed and worked initially, but over time came loose enough to cause problems.

Once installed, run memtest again and verify whether the modules work or not. If you don't get failures immediately, let memtest run for a good 24 hours or better to get some good heat-tolerance testing.

Ok. I'll do it so tomorrow morning and will check every module individually. I suspected not from being different brands but from some other technical detail such as the 64Mx8 - 128Mx8. Thanks for your quick answer.

I've been testing both modules this morning, one by one, and the result was that one gave errors from the first second, and the other is still on test, but without errors for 4 hours. So, apparently, one is defective.

Anyway, the shop now doesn't have availability of those modules, so I will return both and will change them by a pair of exactly the same modules, but in kit (2 x 1 GB, for Dual Channel). In three or four days I hope to have them and test them again.

Your diagnostic, Dngrsone, seems completely correct. Thanks a lot.

No problem, that's what I'm here for.

(contrary to popular belief )

Very useful posting is this.Really it is important for me and i think for others also.
Thanks for this important posting.

No thanks necessary.

In fact, posts like this are generally routed into the trash can.