Since the rig is going to be used mainly for gaming, and your primary concern is FPS, I believe you should ask these questions in a gaming-oriented forum, not a folding@home forum.
Don't ask "What RAM?", because your mobo choice will determine what RAM you should use -- always. You get the RAM that the mobo maker recommends. That's the RAM that the mobo has been tested with. If they say "Crucial 1054Z", then get "Crucial 1054Z", etc. 4GB of RAM is obviously correct. You always want your RAM banks (you'll have two of them), either full, or empty, never half full, always balanced, and never non-interleaved. So you'll have 2 slots per bank, you'll want either 2 slots full of RAM or 4 slots full of RAM, not 1 slot or 3 slots full.
Your should choose a power supply AFTER you choose all your peripherals, especially your video card and mobo. Look and see what power supply will give you: 1) the power you will need, (including the number of rails it uses), with at least a little bit of room for future growth, and 2) the number and type of connectors your mobo and video card will require.
You should know that all power supply makers lie their butts off, (except PC Power & Cooling, and they have been sold, so be sceptical of them, as well). When they're not lying, they're misleading. My Corsair 750 model power supply for instance, will NOT generate 750 watts of power -- of course. (and Corsair is a highly rated power supply brand.)
Good luck with your build, and happy gaming.

If you PC gets bored, I hope you'll check out one of our distributed computing teams: Folding@Home, SETI@Home (bring your tinfoil hat), or Rosetta@Home.