Be careful when adding extra RAM..

[Halvare]

Is the Memory Remapping option enabled in BIOS?

I have Asus MB. When I built the computer and first installed Vista, I did not had good graphics card, because ATI HD2600 cards that I decided to buy, were not available. So I put an old ATI Rage PCI 4MB card into my computer. My computer then recognized only 3.4 GB of RAM even if memory remap was activated. As soon as I bought my HD2600XT, full 4 GB are available.

Don't see any Memory remapping options in the Bios.

And I'm using Vista 64bit Home prem edition.

 

[Sulev Svilponis]

Don't see any Memory remapping options in the Bios.

And I'm using Vista 64bit Home prem edition.

Is your CPU Intel or AMD? What is your MB vendor and model?
Sometimes BIOS update might help.
All MB-s don't have remapping option. Some of those might still do remapping, but if your MB does not support memory remapping, then you can't do anything. If addresses of other system components overlap with RAM addresses and no remapping possible, then some of your RAM is just wasted.