SSD is a much faster alternative.

SSD as the system drive and drive where your windows (or mac) programs are installed will result in very fast bootup times, and near instant launches of your programs. I think that a 120GB minimum SSD drive is sufficient for any serious laptop (or desktop) user. 250GB or greater is recommended if you can afford it.

I also agree with some user's suggestion of using a hybrid setup for cost savings. The system drive should be SSD, and other files should be on the regular HDD.