DGVoodoo shouldn't increase resolutions. It is an API wrapper, not a video driver. It will also be useless under VirtualBox with Win98 as there is no accelerated driver supported for that VM under Win98. Also DGVoodoo only wraps to D3D, in which VirtualBox (in a supported guest like XP) would pass it through the WineD3D wrapper to OpenGL which can have even more variably worse results.
If you want the most resolutions supported in PCem, emulate a Mach64 video card at 4MB. It'll bring you 1280x1024x24 at least. Voodoo 1/2 cards on the other hand are limited to a specific set of resolutions in 16-bit color (512x384, 640x400, 640x480, 800x600 (v1 SLI), 1024x768 (v2 SLI)). Installing DGVoodoo will not change this nor will help as there is no emulated DX9-featured video card.
If there's a guide that involves DGVoodoo being used within an emulator or virtual machine, it's often blind advice/misleading. DGVoodoo works best and is designed for the host OS. It's intended to get older 3d games working on modern systems with modern hardware. Do not confuse DGVoodoo as a driver to use for 3dfx hardware.