What do I need to build a gaming computer?

I want it to mainly to play Sims 2 with all expansion packs, and be somewhat prepared for Sims 3 with a few expansions. I'm looking to spend about 500-800 bucks. What are the components I need to be prepared to buy? What specs should I go with? Best brands? Will it be upgradeable(such as being able to put in a better video card should the one I have become obsolete)? Individual component costs?