"The first thing that comes into our mind is… who needs a 1,500 W power supply anyway? Thousand-watt units are overkill even for the most high-end user, so in the right mind would buy a 1.5 KW unit?" http://www.hardwaresecrets.com/article/594
4 Gb RAM? Have you done research? i7 motherboards have triple channel RAM slots. were you looking to buy 1.33 Gb RAM modules? If you are using 32bit OS, 4Gb will only use just over 3 Gb. If you use Vista 64 bit, get at least 3 x 2Gb RAM.
1500W PSU just makes you look inept, 1000 W will run i7 and 3 top Nvidia cards in SLI.
I concur with everything stated so far - 1.5KW is overkill so divert your cash to something more useful like triple-channel RAM - the money you'd save in getting a 1KW PSU from a 1.5KW PSU would easily upgrade your RAM to a 6GB triple-channel kit.