The one thing to note about the build is that you cant add a second graphics card. You can overclock the i5-2500k and replace the graphics card, but there is no support for dual card setups. I really dont think this is a terrible thing, but i wanted you to be aware.
As others mentioned, its essentially impossible to max the witcher 2 with your budget. this is probably as close as youre going to get, and you will be able to play the game at very high settings, just not the highest.
You wont need a sound card.
If you want to overclock the cpu, you will want an aftermarket cooler, which you can always purchase and install later (for example, after the rebate for your graphics card comes in).