The first versions of Xcode that supported iPhone development required around OS 10.4 or 10.5 (Leopard), if you get a version that could run on a Leopard version you would still not be able to support the last versions of the iPhone OS.
The actual Xcode version you can download requires Snow Leopard and requires you to update your system.
Specific for iPad, as far as I can remember, the first version that supported the iPad required something like 10.6.2 or 10.6.3 (Snow Leopard).
In any case, the advice would be to update your Mac to snow leopard and update it to the last version, if you have the license for snow leopard (around $30) updating your OS is free after that. And what you can download from the developer.apple.com is always the last version (and beta versions if you are an iPhone developer registered).