the problem is the values getting more off with cards over 1GB (1.5GB, 2GB, 3GB etc) and it vary on your normal RAM size > 4GB
MS KB arcicle covers only 1GB case
http://support.microsoft.com/kb/2026022/en-us?p=1
for example i just looked on system with 4GB RAM and 1GB VRAM and it reports in dxdiag
Approx Total memory : 2806 MB
which seems to be correct as
Process Explorer 15.2 (from SysInternals, Microsoft) reports (push CTRL+I)
Dedicated GPU memory = 1014 MB
+
Dedicated System GPU memory = 1792 MB
thats 2806 MB
... now You should compare these 2 values at Your system and see if DXdiag differs
from Process Explorer or if they both off too much from expected/correct value ...