With Mini-ITX, you need to keep one thing in mind: there is only one expansion slot, and that is the one for the graphics card. You won't be physically able to use your soundcard. But that is not so bad, because onboard sound has come a long way and actually quite decent today.
The graphics card you have, should be ok for now. No need to buy a new one. Also space should be not a big problem with a somewhat decent case.
While your psu is a good one, the problem with most mini itx cases is the limited space for the psu. And because the HX850 is longer than a standard psu, it will most likely not fit.
With all the parts you can reuse, your budget needs to cover: CPU, RAM, Motherboard, Case, (optical Drive, PSU)
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: Intel Core i5-4670 3.4GHz Quad-Core Processor ($209.99 @ Newegg)
Motherboard: ASRock B85M-ITX Mini ITX LGA1150 Motherboard ($85.98 @ SuperBiiz)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory ($50.99 @ Newegg)
Case: Fractal Design Node 304 Mini ITX Tower Case ($59.99 @ Newegg)
Power Supply: XFX ProSeries 450W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($59.98 @ SuperBiiz)
Total: $472.93
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-07-25 14:13 EDT-0400)
This is one option. The case has no place for an optical drive (just to point that out).