Upgrade the operating system? OS X 10.5 could be the next step, however, Apple no longer sells it and copies are expensive on the open market.
If your system meets OS X 10.6 specifications, upgrading directly to it might be the better choice and you can upgrade directly from OS X 10.4 to OS X 10.6. This Snow Leopard guide will tell you more about OS X 10.6. To upgrade, call the online Apple store, which in the US is at 800-MY-APPLE, or order OS X 10.6 online at http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard . Install it, do Software Updates until no more are available, which will get you to OS X 10.6.8.
You may need to also upgrade printer drivers and any other non-Apple programs that you use.