depending on the type of license of windows 7 you have, you can or can't activate it to get updates in virtualbox
if it is a retail version, that came in a box, you probably can use it on the pc you prefer, including virtualbox, you will be able to activate it and use it as long as you like
if it is a coa sticker on a case of a pc, it is probable that you won't be able to activate it
virtualbox is really easy to set and use so if you decide to use it, you will find it very fun to use, specially with the hardware you plan to buy
i personally find more useful to have two os in my system, for games and certain apps that will only run properly in windows(wine does alot but not everything yet)
also have one os only for me is like put all your eggs in one basket
pclinux is very hard to break but still i like to have options
i recommend you to have a dualboot system
about the video card, nvidia has the best support on linux so it sounds like a great choice, this model seems to have a small fan, i personally recommend you to get a fanless model if possible, less things to maintain