What cd said - more information here:
http://www.altitude.org/altitude_training.htm

You will never be as fast at a higher altitude as at a lower. There's less oxygen. However, your performance at your old, lower altitude will now be somewhat improved over what it was. As the article states, you would get more benefit at all altitudes by driving to a lower altitude to train.

I find I get noticeable benefit from spending as little as 24-48 hrs. at over 3000m.