Upgrade from 10.2.8 to 10.4

My girlfriend's iMac has OS 10.2.8 and she wants to get to 10.4. Can she do one upgrade? Or does she need to upgrade in multiple steps? Can you suggest where she can get the upgrade software?

thanks.

You have to upgrade, which would mean buying a copy of 10.4 (a.k.a. "Tiger") - either the retail version or one specific to your girlfriend's iMac. What are the specs? Mac OS X v10.4 requires at least 256 MB of RAM. Not totally sure about CPU speed, but I wouldn't install it on anything slower than 500 MHz unless you want it to crawl like a snail.