Retail purchases of Mac OS X 10.4 and new computers that come with Mac OS X 10.4 both include a trial version of iWork. Because of this, installing a full retail copy of iWork is easier than you think.
After purchasing iWork, rather than install the software from the iWork disc, simply open the trial version on your computer. When prompted to buy or try, click the Buy button and enter the serial number that came with your retail copy of iWork. This will unlock the full version—you will no longer be prompted to purchase the software.