Welcome to the forum!
You can purchase Vista then purchase 7 later at upgrade cost but it might be more cost effective to wait til 7 is released and purchase the full 7 install disc. This should work for you, and also if you are purchasing a new pc with Vista preloaded on it, you can wait to purchase until after July 1, 2009 and you will receive a free upgrade to the corresponding Windows 7 version to the preloaded Vista version (i.e. Vista Home Premium-to-7 Home Premium)
Anyways, if you have any questions just ask and we will answer
sceadwe