From what I have learned is 32bit windows/OS's will only address 4GB of RAM. Any hardware you install that has RAM on it will deduct that amount from the total. Video Card 512mb..deduct 512mb. Sound Cards, Hard Drives etc. all get what ever they have deducted from the total of 4GB.
So you start with 4GB add a 512mb Video card and you have 3.5GB.
I think this is pretty close to what happens. But then again, what do I know.
It should be around 3.3gb.