If you have on the host system a graphics card that you can fully dedicate to the guest system, you can then do pci-passthrough and let the guest system have nearly-full access to that graphic card with nearly full performances.
Also, kvm has some nice features that AFAIK virtual box does not have. For example you can have gnu/linux and windows guests use the balloon memory driver, that basically allocates and deallocates memory from the host system. Imagine having a guest vm where you can say "this vm should have 2GB ram, but it might burst up to 24GB if needed, and then go back to 2GB when the need is satisfied". Can't do that with VBox (AFAIK - and it works on windows too!)
Regarding sound, if you are virtualizing windows, imho you're better off connecting via rdp (avoid remmina, use rdesktop) and using local speakers as audio output.