Hi Yessenia,
If your vacation time is flexible, I would try and avoid both June and September (if not possible, go for September). I think the best months to be here are either May or October for a number of reasons.... the weather is much more bearable, airfare from the US is much more reasonable, not fighting the tourist crowds...etc.
The US dollar puts you at an extreme advantage here, but as previously mentioned, gas is expensive and parking, especially in the large cities, is very difficult. Spain has excellent public transportation systems that can get you practically anywhere.
www.renfe.com is the spanish equivalent to amtrak. If you have a PDA (Palm), download the free METRO program which will provide you with access to maps for metro and subway systems for hundreds of cities around the world.
BTW do you know my sister, Jen, in San Francisco?