Hmm I don't think that your card is likely to be fast enough to make use of all the RAM it has. But I'm no expert.
In other news I remember a few threads around a while back saying that the 512Mb GS or maybe another model couldn't be unlocked due to it being a different core version.
There are a few how-tos on overclocking floating around here, look for monkeyman's I think that is one of the best and it's in this topic too. Handy!
