the 1.75gb of ram is allocated to onboard graphics ... i discovered this when doing a recent build... and while installing windows i used the onboard intel HD graphics on the cpu....
while installing windows system informed me i had 4gb of ram with 3.5gb usable ( it varies from motherboard and cpu... it depends how much memory is allocated to the onboard graphics
however if you use a dedicated graphics card ( in my case i used a HD5750 ) you should go into the bios ( press delete at boot ) and disable any options that relate to the onboard graphics...... this include
1) onboard video ... disable
2) init display first... set this to peg or pci-x
depending on the motherboard there may be more options relating to the onboard graphics ( being on the board or on the cpu ) .... consult your motherboard manual
anyway as soon as i started using the HD5750 system preferences reported 4gb installed and 4gb usable ...... happy days
hope this helps