Assalam u Alaikum experts

mere pas 512 mb Nvidia ka vga card hai.
Yeh windows xp mein 512 mb show hota ha or
windows 7 mein 256 mb show hota hai.
Yeh kya masla ha or is ka kya hal hai?
Plz help...
I have
Intel core 2 due 2.66 E6700
4gb ram.