I have been running low on hard disk space for some time and as hard drives are so inexpensive these days I went ahead and bought a 320 gig 2.5" one for my macbook for under $100.
To replace the existing hard drive was so simple as the mac uses built in "time machine" software to backup to an external hard drive.
Below are simple instruction to follow, however they used Super Duper software to copy everything from existing hard drive to external drive.
http://www.macinstruct.com/node/130