iPad compatible with OS X 10.4.11?

You need to have 10.5.8 or later installed for the iPad. Leopard is not a free upgrade. Since Apple no longer produces Leopard you will find it is hard to find. Start by calling AppleCare to see if they still have copies for sale at the original $129.00 price. Otherwise you need to look for a retail copy from third-party vendors such as eBay.

Leopard requires an 867 MHz processor and a minimum of 512 MBs of RAM. If your computer meets the minimum requirements then you should not have a problem.

Leopard is no longer available at the Apple Store *but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753)*.

When you upgrade the OS, do it via Archive & Install. See About the Archive and Install feature http://support.apple.com/kb/HT1710
Be sure to select Preserve Users & Settings. Also, repair permissions before and after the install.

Installing Mac OS X 10.5 Leopard
http://support.apple.com/kb/HT1544

Mac OS X 10.5 Leopard Installation and Setup Guide
http://manuals.info.apple.com/en/leopard_install-setup.pdf

 Cheers, Tom 😉

If the OP goes to 10.5 will he have to change all of his software too? My assumption is that none of the older sw running on 10.4 will work on 10.5+, or am I just delusional?

No, most software will probably be fine and others can be upgraded if needed. Some things like contextual menu items may not work as well as some third-party preference panes, but most of them can be upgraded or replaced.