32-bit windows only recognizes 3-3.5 gigs of ram. You will want to use a 64-bit version.
ATI card can "SLI" as well, except that it's called "crossfire"
An i7-920 + motherboard should be $550ish. 3 x 2gig DDR3 = $150 or so.
If gaming isn't a real priority at the moment I'd go for an HD4850 for $150 and crossfire it later. Even just one can handle any game around at the moment quite well except on the highest resolutions. A very nice PSU and case should be about $100 each. A terrabyte hard drive is $100 or so.
That's all the basic components for around $1150.
Reasonable keyboard/mouse/speakers/dvd-burner should add up to another $100 leaving $350 or so for a nice monitor.
If you're only going to run that one GTX 280 with minimal overclocking, then go with a 750W PSU: Corsair TX750 or PC Power & Cooling Silencer 750.
1000W gives you additional room to both overclock high and SLI the GTX 280. If you want to tri-SLI, then you'd need 1200W.
Whatever power rating you ultimately decide on, go with PC Power & Cooling, Corsair or Antec, in single power rail design (to keep things simple).
Think of how much upgrade headroom you need to build into your PSU, unless you decide to upgrade PSU later if you add more components. This is a tough tradeoff, because PSUs are expensive, should last a long time and carry through more than one build. Suggest you add margin according to budget.
You could go with a cheaper case, but Lian Li's are sweet aluminum. With possibly 4+ hard drives and 2 gtx280's, you'll want a full tower.
You could also go with smaller hard drives, but $75 for a speedy 640gb is pretty cheap. But you'll definitely want two more hard drives in a stripped RAID 0 setup for editing HD video...it's pretty much a requirement for editing HD video.
You'll also need a Blu-Ray burner for the HD videos.
On another note, this is a lot of money (I know nurses make bank, my sis is one) and would be quite the first build. But you may want to start off building a $600-700 PC to get the hang of it and see if you really need to spend $1800+. Not to mention it's fairly easy to sell a cheaper PC on eBay or Craigslist.
$75 - Intel e5200 (overclock it to 3.0ghz+)
$100 - Gigabyte UD3R motherboard
$40 - 4gb DDR2-800
$200 - Nvidia gtx260 http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$50 - Mid tower case of your choice
$40-50 - 450watt+ power supply
$22 - DVD/CD burner
$75 - 640gb hard drive
$120+ - 22" 1680x1050 LCD monitor