Is it possible to install Yosemite in Snow Leopard with Virtualbox?

I'd take the position that you're looking at this the wrong way around.

If Yosemite is going to become your main system (which it is likely to do since it's more up to date), and the main reason for keeping Snow Leopard is so you can run your PPC programs, then your better option is to upgrade to Yosemite and run Snow Leopard in Virtual Box.

There is little (or no?) point in running an old OS -> VirtualBox just to get to the current system. Use VirtualBox for what it's best at (supporting a lesser-used OS image).

Either way, it can be done.

Purchase a fast (Thunderbolt or Firewire 800, depending on the year and model of your Mac) external hard drive and install and run Yosemite from the external hard drive.

I believe this method will allow Yosemite to run faster than from inside a virtual OS environment.

Your Mac still needs to make the minimum system requirements, though.

iMac (Mid-2007 or later)

MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)

MacBook Pro (13-inch, Mid-2009 or later),

MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)

MacBook Air (Late 2008 or later)

Mac mini (Early 2009 or later)

Mac Pro (Early 2008 or later)

Xserve (Early 2009)

Your Mac also needs:

OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed

2 GB or more of memory (I strongly advise, at least, 4 GBs of RAM or more)

8 GB or more of available space

I don't think I suggested a new computer, just running Yosemite as your primary OS, with Snow Leopard in a VirtualBox container for your PPC apps.

Unless you're using your PPC apps a lot (i.e. a majority of the time), this will give you a better experience.

Whichever way you go, though, you might want to consider adding more memory since you'll be running two OSes simultaneously.