The Win 10 TP works in VirtualBox 4.3.12 but VMware Player works better for now. The Virtualbox extensions don't work with the Technical Preview. If you really want to use Virtualbox, be sure to configure settings for Windows 8.1 (64 bit) or (32 bit) depending on which one you are installing and set the Processor to Enable PAE/NX.
The rest of the settings should be already set by default by the Windows 8.1 config, like 2048MB base memory, VT-x/AMD-V enabled, nested paging enabled, video memory 128MB, 3D acceleration enabled, 2D video acceleration enabled.