Decent Future proof Gaming Build?

Hello,
I am looking for a good build for a gaming PC that I can use for the next 3 or 4 years. I am trying to balance the system by how upgradeable it is and the best bang for my buck.

Right now I primarily play WoW. However that will probably change as I am using a 4 year old iMac and my gaming choices have been somewhat limited as of late. I would stay on the darkside but my job now demands that I can support W7 and I have had little to no exposure to it. And I want the freedom to game my brains out. Yes I know bootcamp blah blah but its not the same.

So far I have chosen:

Intel Core i7-950 Bloomfield 3.06GHz 4 x 256KB L2 Cache 8MB L3 Cache LGA 1366 130W Quad-Core Processor BX80601950

ASUS P6X58D-E LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard

Kingston 12GB (3 x 4GB) 240-Pin DDR3 SDRAM DDR3 1066 (PC3 8500) Desktop Memory Model KVR1066D3N7K3/12G

Western Digital Caviar Black WD1002FAEX 1TB 7200 RPM 64MB Cache SATA 6.0Gb/s 3.5" Internal Hard Drive

SPARKLE SXX4602048D5SNM GeForce GTX 460 (Fermi) 2GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card

Now my actual questions.
1 Is 12GB of RAM overkill? I heard 6GB was the sweet spot. But I would rather not have to replace all my ram should that change. Also is the 1066mhz stuff sufficient. From what I have read going to the high end 2000mhz Ram really doesn't show that much of an increase in performance 1-2%. And at over twice the price it doesn't seem like a good buy.

2. How big of a PSU. Keep in mind I may want to slap in an extra GPU maybe 2 later on down the line.

3 And since the MB is SLI capable is it better to buy 1 top of the line video card or 2-3 lesser cards. For instance right now I could buy 2 560 cards for $500 or 1 580 for $500

4 Which I just realized while writing this...
is the gtx 460 or 560 a lot different? I can pick up an ASUS ENGTX560 TI DCII/2DI/1GD5 GeForce GTX 560 Ti (Fermi) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
for about $35 more than the 2gb model of the 460.

Ow at the rate tech moves, just buy as per needs - and for one thing i would hit a GTX 560, HD 6950 or a basement bargain HD 5870 for a beefy gamer hehe