I will be building a new system soon and here is what I want:
CPU: Intel Core i7 2600k MOBO: Asus P8Z68-V PRO/GEN RAM: Kingston HyperX, 4x4GB, DDR3-1600, CL9-9-9-27 SSD: Corsair Force3 240GB GPU: Asus ROG GTX-580 MATRIX 1.5GB Case: Cooler Master 690 II white Powersupply: Seasonic X-760 760W
1.) I could get a i5 2500 but the price difference is not that big and I really multi-task a lot and use Photoshop, Winrar, etc a lot.
2.) I know the MSI GTX-580 Lightning is faster then the Asus ROG Matrix 580 but its 305 mm long, wheres my case can only fit in a 304 mm GPU
3.) I want to use the Seasonic X-760 for a PS but I'm not sure wether it's not too big? 760 Watts sounds like a lot for a single GPU system?
4.) I don't need no other HDD, the 240 GB SSD is enough. There is always external USB 3.0 drives for data you don't use regularly.
I think that you will be able to handle any multitasking that you need with the i5 chip. The sandy bridge chips are so much faster than previous generations that your probably overestimating your needs.
For example, I'm typing this right now on a Macbook Air which as the 1.7ghz i5 chip. I can really can do any multitasking that I want with this computer. Of course my desktop is much faster (i7-920), but this chip is much faster than I thought it would be.
Why don't you take the money you would spend on the i7 and get a bigger case. I've been looking at this corsair cases recently (mostly for their cable managment). This one is also white and looks pretty good. This one will fit a graphics card up to 452mm long (if you remove one of the hard drive bays.