"VirtualBox" is not a generic term. It only refers to one software product, that is VirtualBox.
As far as virtualisation software goes, yes, it's pretty good. You can (as I do) run Adobe CS4 applications in it just fine and the performance is still fairly good too.
While we're talking about virtualisation, I would perhaps suggest not installing Windows 7 as a virtualised "guest". You typically want the lightest possible operating system and for most modern support that's still Windows XP. If you have a license of that, you'll probably find it runs faster.
I'm not sure I'd bother with it for MS Office. Trying to keep yourself locked to Office is usually a recipe for failure. I strongly suggest you attempt a migration to something like OpenOffice or LibreOffice or even something internet-based like Google Documents (which adds on extra collaborative features).
Gaming in a virtualised environment is a no-go. It's just not fast enough. The virtual environment doesn't have direct access to the video hardware (because Ubuntu is using it) so any acceleration is software based (there is a 3D layer provided by Wine - but it's very slow and buggy).
If you want something for gaming you either go with:
Wine. Google for: "appdb your-game-here" and you'll find out how well it is supported. Performance is usually poorer than in Windows and some things just won't run well, if at all. But some games do run really quite well.
Dual Boot. You leave part of your disk dedicated to Windows so that you can still boot to it when you restart. This leaves you with the best gaming performance but it does mean you have to restart to play games and then restart again to get back to Ubuntu.
The important thing you should take away from this is: you don't need to pick just one route for migration. You can have VirtualBox for CS4 apps, native alternatives for Office and Dual boot or Wine for games. Each technology has its benefits and drawbacks and hopefully this will let you choose which is best for each.