Alright, building a computer is a very tough thing, if u care abt saving a few hundred dollars, or if u really want to know the specs and compare them to the rest of the market in order to feel proud of urself for putting all that time in and getting the best value.
1. HDD: You got the S.ATA part right, but i dont get why u would want to spend so much money in order to get a 10,000 RPM HDD. For the same price u can get at least a 250GB when all u get now is a 74GB HDD. It dosent matter as much, performance wise and its a waste of money.
What i would suggest u do, is buy a 250GB HDD from WD, wich make cheaper but more reliable HDDs. You can even find one with a 16M buffer. and all for a little over a hundered dollars.
2. RAM: In order to play today's games, 2GB will do u ok, and u will never need 8GB so getting 2GB in 2 1GB sticks is a good idea (other then getting 2GB sticks). Then u can upgrade to 3, or 4 when u feel like u have to.
3. I see ur looking into an SLi build, wich i think it usually a waste of money unless u spend over 1000$. Also the difference beetween the X1900GT and the X1900XT is huge, and u should always buy the X1900XT. You shouldnt of course buy an X1800 series card, because they are FAR worse then the X1900 series and arent worth it. Of course ATi cards dont overclock so well, and that is usually why a newer series is better (smaller nm process allows for higher clocks), but the X1900 series, and its 48 Fragment Pipelines is a must for anyone buying ATi.
You can find 2 X1900XTs for 600$ if ur lucky, or with mail-in rebate.
But u can also get an Nvidia card. At this point in time, i think that getting a 7900 GT/GTX in Sli is the best thing u can do.