512MB ram but no show

i had 256MB ram i upgraded it to 512MB (sdram pc100/133)

but when i right-click my computer and goto properties it says i only got 382MB ram :(

bad ram or your o/s is very very old and wont take past that

i got win xp!
i think the shop gave me 128MB ram but it says 256sdram on the chip! :(

i will take it back see if i can get a replacement
thx for ya help

okay i changed it at the shop! but still comes up that i got 384MB ram as if i just stuck 128MB ram not 256MB

:(:(
n e thing i can do

Do you have an old PC?? i.e more than 3 years old. And have you tried the stick on its own and see what comes up then?. Chances are if you've an older PC it's not recognising the memory properly (you may need to use PC100 rather than PC133 for example).

i got a recent p.c had it about 2 months
i tried the stick on it own

comes up 128MB ram, the ram i bought is pc133 and the ram that was already in the p.c is pc133