Depends on what you want to do with it...
If you thinking of playing Windows based 3D games, then forget it, use W7 in Bootcamp. If you are just going to use it for less demanding tasks, e.g. office applications and the like, then Parallels will be fine.
I use Oracle VirtualBox to run a Windows XP virtual machine on a quite old MacBook Pro, and it is OK. I have Bootcamp installed with a copy of Windows 7 for more demanding applications, but rarely do I use it.