Well, the S754 is a good idea for you. It definitely gives more performance per $ than the S939. When you decide to upgrade, the S939 won't even be the latest and greatest anymore.
My thoughts about the RAM are this:
Get 2x512 now because it saves money and the 1 GB value sticks run at 3 CL vice 2.5.
If later on you decide you need more, sell one or both 512 sticks then buy the 1 GB sticks. The price will most likely have fallen to close or equal to what 2X512 is.
As for the case fan, I would go with the Antec for the extra air flow. The Sonata doesn't have the best airflow and with a 6800 GT you will want a little extra.