i have a pc with these details

CPU : AMD Athlon 4800 X2
M.B  : Gigabyte GA-M52L-S3
VGA  : Geforce 8600 GT 512 MB
RAM : 2x2Gig DDR2 kingstone =4 Gig
H.D.D : 320 gig maxtor

when i install windows xp sp2 and sp3 32 bit version or windows vista home PREMIUM  32 bit version . windows show 3 Gig RAM in system information whiles i have 4 Gig RAM
i SEE 4 Gig  in RAM information in BIOS section also
I tested these ways:
1) i reduced sum of RAM to 3 and then 2 Gig  and both windows showed 3 Gig and 2 Gig  RAM correctly
2) i tested variant RAMs such as 4x1 gig rams too
3) i changed VGA too but even it did not work
4) i installed a windows 64 bit and it showed 4 Gig RAM correctly
 and i dont know  what must i do  now
have u a suggest for repairing it
 thank you very much 32 bit operating systems can only use a maximum of 4GB of memory.
Memory allocated or sued by graphics cards reduce the total amount that can be seen, also EVERY device uses a bit of RAM too.
In other words, it's a limitation of a 32 bit OS - the maximum you'll ever see is around 3.5GB.
Hope this clears THINGS up.