I have around 4000 photos in iphoto and I know that it is taking up a lot of my memory. i was wondering what is the best form of backing up photos so that they do not always have to be on the computer?
Get an external FireWire hard drive. They are really cost-effective when you consider the long-term cost of repeatedly burning your library to CDs or DVDs. Then you get the added benefit of being able to back up your entire MacBook in the event of a catastrophic failure.
Want to be able to still access your iPhoto library when you're on the go? You can use iPhoto's alias-based mode on the laptop and keep the full files on the external. For more details on how that works, search for Old Toad's tutorials on this subject (alias-based library).
I'll second that. A firewire drive is absolutely the way to go. You can move your iPhoto Library Folder to the drive and make all that space available again on your machine. There's no performance hit either.