If you want a general use computer on which you will sometimes stream video, virtually any computer will do fine.
If you are thinking of dedicating a device solely to streaming videos -- say something you'll attach to your living room TV -- you may be able to get a purpose-built device that will do that job more conveniently and less expensively while also fitting better into a living room type environment. Check out reviews for devices like the Boxee box, the Roku streamer, etc.
You may already have more capabilities than you know: if you have a newer TV, a gaming console like a X360 or PS3, or other TV device like a TiVo, check out those capabilities first. They may have a lot of what you want.