When Mac OS X 10.4 “Tiger” is released on the 29th, it will only be available on DVD media. If your Mac doesn’t have at least a DVD-ROM, you will be unable to use the install disc. Apple’s site says “Tiger ships on a DVD, but if your Mac doesn’t have a built-in DVD-ROM player, you’ll need CD media. When you buy Mac OS X Tiger, you qualify to purchase Tiger CDs for only $9.95.”
Although they say it will be on DVD only, I suspect that it will be possible to use another computer with a DVD drive booted into Target Mode to install 10.4 on a non-DVD Mac. More on this as it becomes available.