I posted here awhile ago when I was planning a good gaming rig which I'd build in a few months. right now I've saved up about $1200 bucks and decided I'll order within this week since my current computer is being a !@#$%. I've chosen the CPU, GPU, PSU, and case. need help with the mobo, CPU cooler, hard drive, optical drive, and anything else I need. btw, it's $1200 excluding tax.
May i recommend Mushkin Redline's (4GB 2x2) for your ramsticks. Not only will you get amazing timings from Greg, but MIR and etc will be EASILY handled.
For the CPU cooler, id recommend Vendetta II paired with a Noctua in a push/pull w/ rear case fans. (grab a backplate mounting for it too (~8$)
MOBO? P5Q Pro? With it u could probably CF the 4870 later if you want to later on.
OS... hmm.. id go Vista x64, but thats personal preference.
When do you plan on purchasing your rig? and where?
If you intend to Crossfire the Radeon 4870 at later date you'll need a more powerful powersupply, especially if you intend on overclocking that pc.
You'll need a minimum 650w or higher the HX620 will do and its modular, but I'd personally recommend the TX750 while its not modular it provides a single 12v rail of 60amps ultimately its whatever you prefer and what suits your budget.
I think getting the TX750 and a single 4870 would be far more economical to start and it won't push you over budget. A 4870x2 plus the HX620 would be more than 60% of your budget and you still have a mess of other components to get.