Parts already selected with descriptively labeled links for parts.
CPU: Pretty sure I'll use a Core i5 2500 or 2500K CPU.
Looking at mid-tower cases with USB 3.0 front panel connectors.
MOBO: Looking for ATX mobo with USB 3 header for front panel USB 3 case. Leaning toward Z68 chipset. Inclined to use Sandybridge GPU now, but might add a cheap video card later, if it helps responsiveness in non-gaming apps.
Since responsiveness is a priority, I wonder if SRT is worth it or not? I hear it can be hard to set up.
I know some here don't like the combo deals but it really does have some nice components. I just built this for myself, added a Cooler Master Hyper 212 Plus and a XFX HD-677X-ZNFC Radeon HD 6770 graphics card. Only reason I added a graphics card was for dual monitor support.
Looks like the combo deal is out of stock, you could just order the individual components might cost a few bucks more. Still should be within your budget.