For an AMD build, to get SLI you have to go for a motherboard with a Nvidia chipset. These are all the ones I could find which are AM3: http://www.newegg.com/Product/ProductList.aspx?Submit=P... Maybe there would be more if I included AM2+. The one's around $100 would probably be my choice if I was going to do what you're planning.
HDD: Either of the following: ~$50/$75 http://www.newegg.com/Product/Productcompare.aspx?Submi...|22-148-395^22-148-395-TS%2C22-152-181^22-152-181-S01%2C22-152-185^22-152-185-TS%2C22-148-433^22-148-433-TS
I suppose if you can find a good combo deal with a Caviar Blue or Caviar Black then they would be worth going for.