Although you can't get iPhoto 6 for free via Software Update, Apple will release minor revisions via Software Update.
To update iPhoto via Software Update:
From the Apple menu, choose System Preferences.
To display the Software Update pane, choose Software Update from the View menu.
To check for updates, make sure you're connected to the Internet, and then click the Check Now button (Figure 1.4).
Figure 1.4. Click Check Now to make Software Update look for iPhoto updates.
If Software Update finds any updates, it launches another application that displays updates that make sense for your Mac, including iPhoto, if any exist.
Select the checkbox next to the update you want to install (Figure 1.5).
Figure 1.5. To install an update, select the checkbox next to its name, and then click the Install button.
Click the Install button, and when the installer prompts you for it, enter your administrator password.
Software Update proceeds to download and install the update with no more input from you.
Choose Check for Updates from the iPhoto application menu at any time to see if you have the latest version of iPhoto.
To set Software Update to kick in on its own, select the Check for Updates checkbox, and from the pop-up menu choose how often you wish it to check.
When Software Update fails to find updated software appropriate for your computer, it tells you none are available.
Watching for Updates
I recommend setting Software Update to check for updates automatically, and iPhoto can do so itself too (select Check for iPhoto Updates Automatically in the General pane of iPhoto's Preferences window). You can also visit Apple's iPhoto Web pages at www.apple.com/iphoto/ every so often for news about iPhoto or updates that might not have been released via Software Update.