I don't think it is the OS that causes the lagging as much as the components do. I would go with Vista 64bit as the best. If you are planning on building a computer or putting a new OS onto an existing computer you can get deals right now on Newegg which for 10 dollars more on Windows Vista Home Premium 64bit you can get a free upgrade coupon to Windows 7 when it is released. So basically you are getting Windows 7 for only 10 dollars.
It is also good for those who have vista now because you can basically get Windows 7 for only 109.99. But you will have to do a clean install of vista now which might not be very good for some people. You can also get 50% off of a windows 7 upgrade if you pre purchase it now.