There's a few possible explanations for your problem :
Most likely - the chip configuration of the new modules aren't supported by your motherboard. The 128x4 spec means that the new modules use high density chips & your motherboard might not support these.
Also, if you have a big brand computer (eg HP or similar), then these tend to be far more fussy about the type of ram they'll work with.
Possible but unikely explanations :
1) Your system doesn't support 1GB modules. This is not likely to be the reason however, as normally the system will work, but it will recognise the modules as 512MB instead of 1GB for example.
2) The new modules are registered (buffered) ram. Unlikely because these are hard to get hold of, a lot more expensive, plus there's nothing in the model number to suggest that this is the case.
I suggest you try this :
Go to crucials website - www.crucial.com
Make sure you select the country you live in (i.e UK, USA or Canada) then use Crucials system scanner to find out what computer you have. If you have a big brand machine, then it should tell you the model of the computer. If you have a generic or custom built system, then it should tell you what the motherboard is. It will also give you a list of ram that your system supports.
If the system scanner doesn't work, then download belarc Advisor : http://www.belarc.co...e_download.html
Use this to find out which motherboard you have.
Whichever method you use, let me know what the motherboard is.