First time builder need your help

HI,
I have decided to build my own gaming machine, but since i have absolutely no experience in teh matter whatsoever, I have a few questions:
first here is what I intend to get:

Approximate Purchase Date: before january 15

Budget Range: 1000-1700 GBP

System Usage from Most to Least Important: gaming, working (word, excel, powerpoint, surfing the internet, watching movies)

Parts Not Required: keyboard, mouse, monitor, speakers

Preferred Website(s) for Parts: amazon.co.uk

Country: UK

Parts Preferences:
core i5 2500k
asus p8z68 V Pro/Gen 3
Asus GeForce GTX 580 DirectCu II Graphics Card
or 2 Asus 1GB GeForce GTX 560TI DirectCUII in SLI
Sony AD-7261S-0B 24x Internal DVDRWRAM SATA Black Lightscribe
Microsoft Windows 7 Professional
Crucial CT064M4Solid State Drive2 64GB M4 Solid State Drive for SRT
Corsair CML16GX3M4A1600C9B LP Vengeance 16GB 1600MHz CL9 DDR3 Four Module Kit or the 8Gb kit of the same brand
Cooler Master HAF X - Full tower
Corsair CMPSU-750AXUK Professional Series Gold 750W Power Supply
Hitachi Hds721010cla332 3.5inch 1tb 32mb Sata 7200rpm Hard Disk Drive
or Seagate ST31000524AS 3.5 inch Barracuda 1TB GB 7200rpm SATA Drive with 32MB Buffer

Overclocking: Yes

SLI or Crossfire: Maybe

Monitor Resolution: 1920x1200

Additional Comments:I would like a quiet PC if possible

please feel free to comment on those choices.
now the specific questions:
1)I plan to overclock the CPU and the GPU ( i won't push the limits though, i just want to get a feel at overclocking and increased performance)
Will the PSU chosen be enough even if I SLI 2 GTX 580 in the future?
2)DO i need cpu cooler or can I getaway with just a heatsink and the haf X fans?
3a)If i need a cooler which one? I have read that most air cooler do interfere with one of the memory slot on the motherboard, especially the vengeance with high heatsink.
3b)Since I am going for the low profile RAM, which aircooler would definitely work even with 16gb of ram installed?
4)should i go for 8gb RAM or 16Gb?

Thank you