Well, let's gets some facts out of the way to back up my opinions.RAM usage
I've come to find that Windows will scale RAM usage to about 20%-25%, using machines that had 4GB, 6GB, and 8GB of RAM. So you still have plenty of space for your other applications should you run them. Most applications I've also used, which aren't by any means heavy hitters, rarely peak over 500MB of usage. The only one that did was Firefox, but Mozilla seems to have mitigated its RAM hungriness a lot in recent versions.
There's also the fact that most games are still 32-bit executables. In a 64-bit Windows environment, Windows, by default, allows only 2GB of RAM to be used. Once that 2GB limit is reached, the program thinks it's run out of memory. There's a way for the program to use all 4GB available in a 32-bit environment, but this requires a recompile of the program itself. Very few games are compiled in this way, and as far as I know, the only one that is is Skyrim. Plus 64-bit programs are actually kind of rare still. I only have a handful of 64-bit programs installed.Power Supply
Believe it or not, 550W-600W is actually all you need for a single video card setup, for the rest of its life. If you want to go SLI, then I would certainly suggest a beefier one, but this has to be a short term decision. I don't like SLI anymore, I've had it, all it did was suck more power and made more fan noise. And also, when you do upgrade, you have two cards to get rid of.
For my system I have a Core i5-2500 and a GTX 670. Under a reasonable load such as a game, the power draw from the socket is only 230W. Even if I were to upgrade to a future video card, I wouldn't expect to see much of a power draw difference anyway as successive shrinking of transistors allow for more computational power for less electrical power. Heck, this is the same amount of power my last gaming PC used, which sported a Core 2 Duo with a 8800GT 512MB.
Given what I measured, I'm also fairly confident that I could even run SLI off a 600W power supply. But I'd rather not.
If you want to know how much power you need, plug in everything you're going to throw on the system at http://extreme.outervision.com/PSUEngine
. Since it calculates by default 90% load. Just take what it says, round up or down to the nearest 50W, and add half of it back. So if it recommends 390W, round up to 400W, then add 50% of 400 (200) and... you get 600W.Video card
Tom's Hardware updated their best graphics card for the money
views (URL is for your price range). They do this monthly too.
Also I should update my recommendation and it's gonna sound confusing. I would recommend the Radeon HD7970 GHz Edition (not the regular one) if you're going to spend that much on a video card. Otherwise if we go down the $400 or so range the Radeon HD7970 does better than the GTX 670. However, don't go above $450 on a video card unless you are driving resolution games at 2560x1400 or higher, or are running one of those multi-monitor gaming setups (three-screen or six-screen). In fact, if we were to be even more economical, the GTX 660 Ti and Radeon HD7870 will run all games at 1080p on maximum detail just fine.