GPU limited to 64mb of ram

Got an Acer Aspire 5050 the other day, and this thing is pretty damn nice for it's price. The thing that bothers me is that fact that the GPU can use up to 256mb of RAM (shared), but, seeing as the lappy only has 512 it limits itself to 64... Which with most games, simply doesn't cut it (try playing HL2.. lol). I want it to use 256mb... It's MY laptop, why the hell can't I?? >.<

Anyone know of a way to change it? I mean, i know it's not smart to let it use that much RAM, but it would only be for when i'm gaming, then i'd change it back.

Thanks.

what graphics card do you have?

Oh, forgot to paste the specs. Here you go:

ATI Radeon® Xpress 1100 chipset