I assume we will see 512MB video cards hit the market widespread in May, at the time of the ATi r520 release. nVidia will most certainly follow suit.
What is your definition of big? This
may not be a WD Raptor, but it can certainly keep up in speed easily and by today's standard is in no way small.
Not sure about the FX-57, but I can safely say it'll be out in no more than a year from now, the FX-55 has been out for quite a while now...
As for your final question, I'm quite certain that one the price of DDR2 drops, which I have heard will happen in the next few months sometime, AMD will migrate over to it. Any socket 939 board now should be able to support the FX-57, and 8GB of memory is simply ridiculous, but I have heard that dual cores will be able to handle a llot more RAM.